Add tests and documentation for fetch command

- Add test_cli.py with 6 tests covering:
  - Directory structure creation
  - File naming convention
  - JSON content (account metadata + transactions)
  - Duplicate account name handling
  - Required options validation

- Update README.md with fetch command documentation

Author: robcohen

README.md

@@ -204,3 +204,52 @@ cog.out(
 }
 ```
 <!-- [[[end]]] -->
+
+#### Fetch all accounts to separate files
+
+`simplefin fetch --output-dir DIRECTORY [--lookback-days INTEGER]`
+
+Fetches all accounts with their transactions and saves each account to a separate JSON file. Files are organized hierarchically by institution and account name:
+
+```
+output-dir/
+  institution-domain/
+    account-name/
+      account-id_YYYY-MM-DD.json
+```
+
+This is useful for integration with tools like [beangulp](https://github.com/beancount/beangulp) that expect one file per account.
+
+```
+❯ simplefin fetch --output-dir ./simplefin-data --lookback-days 30
+Found 2 accounts
+  SimpleFIN Savings: 3 transactions -> beta-bridge.simplefin.org/SimpleFIN-Savings/Demo-Savings_2025-01-15.json
+  SimpleFIN Checking: 2 transactions -> beta-bridge.simplefin.org/SimpleFIN-Checking/Demo-Checking_2025-01-15.json
+
+Wrote 2 account files to ./simplefin-data
+```
+
+Each JSON file contains the full account metadata plus transactions:
+
+```json
+{
+  "org": {
+    "domain": "beta-bridge.simplefin.org",
+    "name": "SimpleFIN Demo"
+  },
+  "id": "Demo Savings",
+  "name": "SimpleFIN Savings",
+  "currency": "USD",
+  "balance": "115525.50",
+  "balance-date": 1738368000,
+  "transactions": [
+    {
+      "id": "1738382400",
+      "posted": "2025-02-01T12:00:00+00:00",
+      "amount": "-50.00",
+      "description": "Fishing bait",
+      "payee": "John's Fishin Shack"
+    }
+  ]
+}
+```

tests/test_cli.py

@@ -0,0 +1,236 @@
+import datetime
+import json
+import os
+import tempfile
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+from click.testing import CliRunner
+
+from simplefin.cli import cli
+
+
+@pytest.fixture
+def mock_accounts():
+    """Sample accounts response."""
+    return [
+        {
+            "org": {
+                "domain": "beta-bridge.simplefin.org",
+                "name": "SimpleFIN Demo",
+            },
+            "id": "ACT-savings-123",
+            "name": "SimpleFIN Savings",
+            "currency": "USD",
+            "balance": "1000.00",
+            "balance-date": 1736553600,
+        },
+        {
+            "org": {
+                "domain": "beta-bridge.simplefin.org",
+                "name": "SimpleFIN Demo",
+            },
+            "id": "ACT-checking-456",
+            "name": "SimpleFIN Checking",
+            "currency": "USD",
+            "balance": "500.00",
+            "balance-date": 1736553600,
+        },
+    ]
+
+
+@pytest.fixture
+def mock_transactions():
+    """Sample transactions response."""
+    return [
+        {
+            "id": "TRN-001",
+            "posted": datetime.datetime(2025, 1, 10, 12, 0, 0),
+            "amount": "-50.00",
+            "description": "Test transaction",
+            "payee": "Test Payee",
+        },
+    ]
+
+
+class TestFetchCommand:
+    """Tests for the fetch CLI command."""
+
+    def test_fetch_creates_directory_structure(self, mock_accounts, mock_transactions):
+        """Test that fetch creates the correct directory structure."""
+        runner = CliRunner()
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            with patch.dict(os.environ, {"SIMPLEFIN_ACCESS_URL": "https://mock"}):
+                with patch("simplefin.cli.SimpleFINClient") as MockClient:
+                    mock_client = MockClient.return_value
+                    mock_client.get_accounts.return_value = mock_accounts
+                    mock_client.get_transactions.return_value = mock_transactions
+
+                    result = runner.invoke(
+                        cli,
+                        ["fetch", "--output-dir", tmpdir, "--lookback-days", "7"],
+                    )
+
+                    assert result.exit_code == 0
+                    assert "Found 2 accounts" in result.output
+
+                    # Check directory structure
+                    inst_dir = Path(tmpdir) / "beta-bridge.simplefin.org"
+                    assert inst_dir.exists()
+
+                    savings_dir = inst_dir / "SimpleFIN-Savings"
+                    checking_dir = inst_dir / "SimpleFIN-Checking"
+                    assert savings_dir.exists()
+                    assert checking_dir.exists()
+
+    def test_fetch_creates_json_files_with_correct_naming(
+        self, mock_accounts, mock_transactions
+    ):
+        """Test that JSON files are created with correct naming convention."""
+        runner = CliRunner()
+        today = datetime.date.today().isoformat()
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            with patch.dict(os.environ, {"SIMPLEFIN_ACCESS_URL": "https://mock"}):
+                with patch("simplefin.cli.SimpleFINClient") as MockClient:
+                    mock_client = MockClient.return_value
+                    mock_client.get_accounts.return_value = mock_accounts
+                    mock_client.get_transactions.return_value = mock_transactions
+
+                    result = runner.invoke(
+                        cli,
+                        ["fetch", "--output-dir", tmpdir, "--lookback-days", "7"],
+                    )
+
+                    assert result.exit_code == 0
+
+                    # Check file naming
+                    savings_file = (
+                        Path(tmpdir)
+                        / "beta-bridge.simplefin.org"
+                        / "SimpleFIN-Savings"
+                        / f"ACT-savings-123_{today}.json"
+                    )
+                    assert savings_file.exists()
+
+    def test_fetch_json_contains_account_and_transactions(
+        self, mock_accounts, mock_transactions
+    ):
+        """Test that JSON files contain both account metadata and transactions."""
+        runner = CliRunner()
+        today = datetime.date.today().isoformat()
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            with patch.dict(os.environ, {"SIMPLEFIN_ACCESS_URL": "https://mock"}):
+                with patch("simplefin.cli.SimpleFINClient") as MockClient:
+                    mock_client = MockClient.return_value
+                    mock_client.get_accounts.return_value = mock_accounts
+                    mock_client.get_transactions.return_value = mock_transactions
+
+                    result = runner.invoke(
+                        cli,
+                        ["fetch", "--output-dir", tmpdir, "--lookback-days", "7"],
+                    )
+
+                    assert result.exit_code == 0
+
+                    # Read and verify JSON content
+                    savings_file = (
+                        Path(tmpdir)
+                        / "beta-bridge.simplefin.org"
+                        / "SimpleFIN-Savings"
+                        / f"ACT-savings-123_{today}.json"
+                    )
+
+                    with open(savings_file) as f:
+                        data = json.load(f)
+
+                    # Check account metadata
+                    assert data["id"] == "ACT-savings-123"
+                    assert data["name"] == "SimpleFIN Savings"
+                    assert data["currency"] == "USD"
+                    assert data["balance"] == "1000.00"
+
+                    # Check transactions were merged
+                    assert "transactions" in data
+                    assert len(data["transactions"]) == 1
+                    assert data["transactions"][0]["id"] == "TRN-001"
+
+    def test_fetch_handles_duplicate_account_names(self, mock_transactions):
+        """Test that accounts with the same name get unique files."""
+        runner = CliRunner()
+        today = datetime.date.today().isoformat()
+
+        # Two accounts with the same name but different IDs
+        accounts_with_duplicates = [
+            {
+                "org": {"domain": "example.com", "name": "Example Bank"},
+                "id": "ACT-111",
+                "name": "Checking",
+                "currency": "USD",
+                "balance": "100.00",
+                "balance-date": 1736553600,
+            },
+            {
+                "org": {"domain": "example.com", "name": "Example Bank"},
+                "id": "ACT-222",
+                "name": "Checking",
+                "currency": "USD",
+                "balance": "200.00",
+                "balance-date": 1736553600,
+            },
+        ]
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            with patch.dict(os.environ, {"SIMPLEFIN_ACCESS_URL": "https://mock"}):
+                with patch("simplefin.cli.SimpleFINClient") as MockClient:
+                    mock_client = MockClient.return_value
+                    mock_client.get_accounts.return_value = accounts_with_duplicates
+                    mock_client.get_transactions.return_value = []
+
+                    result = runner.invoke(
+                        cli,
+                        ["fetch", "--output-dir", tmpdir, "--lookback-days", "7"],
+                    )
+
+                    assert result.exit_code == 0
+
+                    # Both files should exist in the same directory
+                    checking_dir = Path(tmpdir) / "example.com" / "Checking"
+                    files = list(checking_dir.glob("*.json"))
+                    assert len(files) == 2
+
+                    # Files should have different account IDs in names
+                    file_names = [f.name for f in files]
+                    assert f"ACT-111_{today}.json" in file_names
+                    assert f"ACT-222_{today}.json" in file_names
+
+    def test_fetch_requires_output_dir(self):
+        """Test that --output-dir is required."""
+        runner = CliRunner()
+
+        with patch.dict(os.environ, {"SIMPLEFIN_ACCESS_URL": "https://mock"}):
+            result = runner.invoke(cli, ["fetch"])
+
+            assert result.exit_code != 0
+            assert "Missing option '--output-dir'" in result.output
+
+    def test_fetch_requires_access_url_env(self):
+        """Test that SIMPLEFIN_ACCESS_URL environment variable is required."""
+        runner = CliRunner()
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            # Ensure env var is not set
+            env = os.environ.copy()
+            env.pop("SIMPLEFIN_ACCESS_URL", None)
+
+            with patch.dict(os.environ, env, clear=True):
+                result = runner.invoke(
+                    cli,
+                    ["fetch", "--output-dir", tmpdir],
+                )
+
+                # Should fail because no access URL
+                assert result.exit_code != 0