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.
