docs: add portable LLM workflow skill

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

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+# Review PR Prompt
+
+You are independently reviewing PR `<pr-number>` in `<repo-name>`.
+
+Hard gate:
+
+- Verify authenticated identity.
+- Verify PR author.
+- If authenticated user matches the PR author, stop immediately.
+- Do not approve or merge unless reviewer-eligible.
+
+Workflow:
+
+1. Create or use a separate review worktree.
+2. Fetch/prune.
+3. Confirm PR `<pr-number>` is open and targets `<default-branch>`.
+4. Confirm the PR head is `<expected-commit>` or includes it.
+5. Confirm changed files match the issue scope.
+6. Inspect the diff for correctness, secrets, unrelated behavior, and forbidden
+   changes.
+7. Run `<required-checks>`.
+8. If anything fails, leave exact blockers and do not approve.
+9. If everything passes, approve only if reviewer-eligible.
+
+Report:
+
+- authenticated user
+- PR author
+- eligibility result
+- commit reviewed
+- files changed
+- checks run and results
+- blockers or risks
+- final PR URL