Agent Skills: pr-precheck

Pre-submission due diligence for upstream PRs to public/large repositories. Use this before opening a PR — especially in repos with deep PR backlogs (hundreds+ open) — to avoid duplicating existing work, sidestepping the actual root cause, or building on stale or deprecated architecture. Triggers on phrases like "提个 PR", "want to contribute", "open a PR upstream", "fix this in <repo>", "PR 前的尽职调查". See references/checklist.md for the full procedure and references/gh-recipes.md for ready-to-paste search commands.