fix: pr self-review cleanup

* remove duplicate merge migration note from README Known Limitations
* preserve exception chain with `from exc` in generate_sql
* handle missing git binary in file_exists_on_branch
* add .env to .gitignore

Author: stephen-kintsugi

.gitignore

@@ -25,5 +25,8 @@ htmlcov/
 .vscode/
 .DS_Store
 
+# Environment
+.env
+
 # Python version
 .python-version

README.md

@@ -64,7 +64,6 @@ Merge migrations (where `down_revision` is a tuple) are skipped since they produ
 * The hook runs `alembic upgrade --sql`, which executes your project's `env.py` in offline mode. No database connection is made, but the Python code in `env.py` does run.
 * If `DATABASE_URL` is not set, the hook provides a dummy fallback (`postgresql://localhost/lint`) so alembic's offline mode can generate SQL without a real connection string.
 * If `alembic upgrade --sql` fails for a migration (e.g. due to missing dependencies or env configuration), the hook prints the error to stderr and fails the run.
-* Merge migrations (where `down_revision` is a tuple) produce no DDL and are always skipped.
 
 ## Squawk Configuration
 

squawk_alembic/hook.py

@@ -121,10 +121,10 @@ def generate_sql(filepath):
             text=True,
             env=env,
         )
-    except FileNotFoundError:
+    except FileNotFoundError as exc:
         raise GenerateSqlError(
             "squawk-alembic: alembic not found. Ensure alembic is installed in your environment."
-        )
+        ) from exc
 
     if result.returncode != 0:
         raise GenerateSqlError(
@@ -161,10 +161,13 @@ def validate_branch(branch):
 
 def file_exists_on_branch(filepath, branch):
     """Check if a file exists on the given git branch."""
-    result = subprocess.run(
-        ["git", "cat-file", "-e", f"{branch}:{filepath}"],
-        capture_output=True,
-    )
+    try:
+        result = subprocess.run(
+            ["git", "cat-file", "-e", f"{branch}:{filepath}"],
+            capture_output=True,
+        )
+    except FileNotFoundError:
+        return False
     return result.returncode == 0