chore: add pull-requests agent skill (#23364)

Adds a repo-local agent skill at `.agents/skills/pull-requests/SKILL.md`
that guides agents through the PR lifecycle for this repository:
creating, updating, and following up on pull requests.

Covers lifecycle rules (reuse existing PRs, default to draft), local
validation commands (`make pre-commit`, `make lint`, etc.), PR
title/description conventions, CI check follow-up, and explicit
guardrails against common mistakes.

.agents/skills/pull-requests/SKILL.md

@@ -0,0 +1,72 @@
+---
+name: pull-requests
+description: "Guide for creating, updating, and following up on pull requests in the Coder repository. Use when asked to open a PR, update a PR, rewrite a PR description, or follow up on CI/check failures."
+---
+
+# Pull Request Skill
+
+## When to Use This Skill
+
+Use this skill when asked to:
+
+- Create a pull request for the current branch.
+- Update an existing PR branch or description.
+- Rewrite a PR body.
+- Follow up on CI or check failures for an existing PR.
+
+## References
+
+Use the canonical docs for shared conventions and validation guidance:
+
+- PR title and description conventions:
+  `.claude/docs/PR_STYLE_GUIDE.md`
+- Local validation commands and git hooks: `AGENTS.md` (Essential Commands and
+  Git Hooks sections)
+
+## Lifecycle Rules
+
+1. **Check for an existing PR** before creating a new one:
+
+   ```bash
+   gh pr list --head "$(git branch --show-current)" --author @me --json number --jq '.[0].number // empty'
+   ```
+
+   If that returns a number, update that PR. If it returns empty output,
+   create a new one.
+2. **Check you are not on main.** If the current branch is `main` or `master`,
+   create a feature branch before doing PR work.
+3. **Default to draft.** Use `gh pr create --draft` unless the user explicitly
+   asks for ready-for-review.
+4. **Keep description aligned with the full diff.** Re-read the diff against
+   the base branch before writing or updating the title and body. Describe the
+   entire PR diff, not just the last commit.
+5. **Never auto-merge.** Do not merge or mark ready for review unless the user
+   explicitly asks.
+6. **Never push to main or master.**
+
+## CI / Checks Follow-up
+
+**Always watch CI checks after pushing.** Do not push and walk away.
+
+After pushing:
+
+- Monitor CI with `gh pr checks <PR_NUMBER> --watch`.
+- Use `gh pr view <PR_NUMBER> --json statusCheckRollup` for programmatic check
+  status.
+
+If checks fail:
+
+1. Find the failed run ID from the `gh pr checks` output.
+2. Read the logs with `gh run view <run-id> --log-failed`.
+3. Fix the problem locally.
+4. Run `make pre-commit`.
+5. Push the fix.
+
+## What Not to Do
+
+- Do not reference or call helper scripts that do not exist in this
+  repository.
+- Do not auto-merge or mark ready for review without explicit user request.
+- Do not push to `origin/main` or `origin/master`.
+- Do not skip local validation before pushing.
+- Do not fabricate or embellish PR descriptions.