fix: add checkout validation contract to worker tasks

[fujiwaranosai850]

Addresses issue #171

Summary

• add checkout-contract context to dispatched developer, tester, and reviewer task messages
• derive canonical target branch, ref, commit, and expected worktree path before dispatch
• update default worker prompts so decisive validation requires a clean isolated checkout
• add a targeted message-builder test for checkout-contract rendering

Validation

• npx vitest run lib/dispatch/message-builder.test.ts
• npm run check (currently fails on pre-existing repo test typing/module issues unrelated to this branch)