test(cli): add comprehensive v2 CLI integration test suite (#304)

* test(cli): add comprehensive v2 CLI integration test suite

57 tests across 15 classes covering the full CLI surface area:
init, status, summary, PRD, tasks, work, batch, blocker,
checkpoint, patch, schedule, templates, review, and golden-path E2E.

Uses CliRunner against real SQLite databases (no mocks).

* test(cli): add AI integration tests with MockProvider

6 new tests exercise the LLM code paths through the CLI:
- Task generation via LLM (with valid JSON, overwrite, and fallback)
- Agent execution dry-run (planner + executor)
- Agent execution creating a file (verifies file on disk)
- Full AI golden path E2E (generate → plan → execute)

Uses MockProvider injected via monkeypatch on get_provider,
with canned plan JSON and file content responses.

* fix(lint): remove unused Path import

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Co-authored-by: Test User <test@example.com>

Author: frankbria