test: add Vitest test infrastructure and baseline test coverage

[teckedd-code2save]

Summary

Add Vitest test infrastructure to Convoy and establish baseline test coverage for core stage execution and state management.

Why

• Convoy is a deployment agent that writes files, creates PRs, and deploys to Fly.io — operations that are risky to test manually
• The project has zero automated tests and zero open issues, which means quality gaps are undetected
• The deterministic core (scanner, picker, author, orchestrator, state store, medic) is well-isolated and highly testable with mocked adapters
• Tests would prevent regressions in the SQLite state machine, plan execution pipeline, and adapter interfaces
• The project already has a clean TypeScript setup — adding Vitest requires minimal boilerplate

What to Do

1. Install Vitest: Add vitest as a dev dependency. Add a test script to package.json.

2. Create test config: A vitest.config.ts that excludes web/ and demo-app/ from the test scope.

3. Prioritize core modules for initial coverage:

   • src/core/state.ts — SQLite state store operations (create plan, transition stages, query history). Test schema migration and CRUD operations.
   • src/core/stages/ — Each stage (scan, plan, author, rehearse, canary, promote, observe, medic) should have a test verifying its interface contract and state transitions.
   • src/planner/scanner.ts — File tree scanning logic. Test with fixture directories.
   • src/planner/author.ts — Template-based file authorship. Test that templates produce valid output.
   • src/adapters/fly/runner.ts — Test with mocked flyctl responses. Verify error handling and rollback logic.

4. CI integration: Update .github/workflows/ci.yml to run npm test on every push and pull request.

Acceptance Criteria

• vitest in devDependencies
• vitest.config.ts exists at repo root
• npm test runs Vitest with at least 3 passing tests
• Tests cover state store CRUD operations (create plan, read, transition)
• Tests cover scanner file tree detection with a fixture directory
• Existing CI workflow runs tests
• Tests are deterministic (no network calls, no external dependencies)