test: add test infrastructure and initial test suite

[teckedd-code2save]

Summary

Convoy is a core Serendepify product — a deterministic deploy agent with real stage runners (GitHub PR creation, Fly.io deploy, local rehearsal). Despite its production-grade features, the repo has no test infrastructure or test files.

What to do

1. Add a test runner (e.g., vitest — used by other repos in the org) to the root package.json
2. Write unit tests for the core modules:
   • src/core/ — orchestrator, state store, event bus, medic
   • src/planner/ — scanner, picker, author (especially the template logic)
   • src/adapters/fly/runner.ts — shell wrappers (mock flyctl)
3. Add integration tests for the rehearsal runner (src/core/rehearsal-runner.ts) with a mock target
4. Configure the existing ci.yml workflow to run npm test (or npm run typecheck && npm test)
5. Consider npx vitest --coverage for coverage reporting

Why it matters

• Regression protection: Real stage runners execute shell commands and modify state. Without tests, a refactor or new adapter silently breaks existing flows.
• CI gates: Tests block bad merges. The CI workflow exists but only does typechecking — adding tests gives us a proper safety net.
• Onboarding: New contributors (human or AI) rely on npm test to validate their changes. Missing tests means every change is a blind push.

Acceptance criteria

• npm test (or npm run test) passes with at least one test file
• Core planner modules have unit tests
• CI workflow runs npm test
• A tests/ directory exists at repo root with at least 3 test files covering real logic