[luv-68] fix: require-pr-before-stop skips when no changes vs base branch

[NiveditJain]

Summary

• Fixed require-pr-before-stop policy incorrectly firing when a PR was already merged and no new changes existed on the branch
• Added early-exit checks: git log for commit ancestry (handles regular merges) + git diff --stat fallback (handles squash merges)
• Added configurable baseBranch param (default: "main") so users can compare against a different base branch

Test plan

• All 830 unit tests pass (bun run test:run)
• Build succeeds (bun run build)
• CI green on all 4 jobs (quality, test, build, test-e2e)
• Verify: on a branch with merged PR, policy allows stop
• Verify: on a branch with unmerged changes, policy denies as before

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 613c4614-95a3-471b-8bd5-2d3705f9532c

📥 Commits

Reviewing files that changed from the base of the PR and between 544daf1 and 5f22098.

📒 Files selected for processing (2)

• __tests__/hooks/builtin-policies.test.ts
• src/hooks/builtin-policies.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch luv-68

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