docs: add portable LLM workflow skill

skills/llm-project-workflow/templates/merge-pr.md

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+# Merge PR Prompt
+
+You are the merge operator for PR `<pr-number>` in `<repo-name>`.
+
+Hard gate:
+
+- Verify authenticated identity before reviewing or merging.
+- Verify PR author.
+- If authenticated user matches the PR author, stop immediately.
+- Merge only if assigned merge duty and reviewer-eligible.
+
+Workflow:
+
+1. Confirm PR `<pr-number>` is open and targets `<default-branch>`.
+2. Confirm the PR head is still `<reviewed-commit>` or includes it.
+3. Confirm changed files still match the reviewed file list.
+4. Confirm previous validation still applies or rerun required checks.
+5. Merge through the gated project merge workflow.
+6. Confirm remote `<default-branch>` contains the merge.
+7. Close or release linked issue `<issue-number>` according to project policy.
+8. Remove `status:in-progress` if applicable.
+9. Delete the merged remote branch if policy allows.
+10. Remove local PR branch and branch worktree only after merge is confirmed.
+11. Fetch/prune.
+12. Confirm main checkout is clean/current.
+
+Stop if:
+
+- authenticated user is the PR author
+- PR head changed after review
+- PR is closed but not merged
+- tests/checks fail
+- cleanup would delete unmerged work
+
+Report:
+
+- authenticated user
+- PR author
+- eligibility result
+- merge commit/hash
+- issue final state
+- branch/worktree cleanup result
+- final PR URL