GitHub PR Review Agent — System Prompt + Workflow

Two files that give every PR a senior review: (1) a staff-engineer system prompt with a strict rubric — correctness, security (any hit auto-BLOCKER), test coverage, complexity, naming, consistency — severity-tagged findings with file:line citations and suggested diffs, mandatory praise for good decisions, and an oversized-diff protocol; (2) a ready GitHub Actions workflow that runs it on every PR open/update and posts the review as a comment, with a skip-ai-review label escape hatch and concurrency control. Setup: one repo secret, two file commits.