[luv-198] fix: skip no-conflicts policy when no OPEN PR exists

[NiveditJain]

Summary

• require-no-conflicts-before-stop used to run its local git merge-tree probe (Layer 1) unconditionally, so a branch with no PR on GitHub — or only a closed/merged PR — could still receive a deny that referenced a PR URL the user never created.
• Promoted the gh + PR-state precheck above Layer 1: when gh is unavailable, no PR exists, or pr.state !== "OPEN", the policy short-circuits to allow and never runs Layer 1. Layer 2 reuses the pr already fetched in the precheck — no second gh pr view call.
• Per user direction: when gh is not installed we skip the entire policy (without gh we cannot confirm a PR exists).

Test plan

• bun run test:run — 991 unit tests pass
• bun run test:e2e — 207 e2e tests pass
• bunx tsc --noEmit — clean
• bun run lint — only a pre-existing unrelated warning
• New unit tests assert Layer 1 is not invoked (using mergeTreeStatus: 1) when gh is missing / no PR / PR is CLOSED
• Manual repro: check out a branch with no PR on GitHub, trigger the Stop hook, confirm the policy now allows with "No pull request found for branch, skipping conflict check." and emits no URL

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 51fa0a64-30a7-4be3-b3e7-f23163a38fd2

📥 Commits

Reviewing files that changed from the base of the PR and between 7081187 and de0e96f.

📒 Files selected for processing (4)

• CHANGELOG.md
• __tests__/hooks/builtin-policies.test.ts
• docs/built-in-policies.mdx
• src/hooks/builtin-policies.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch luv-198

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