Protect Python from AI slop PRs

[kimjune01]

7 of 61 PRs in the last 2 weeks would have been auto-closed before review.

Install a GitHub Action:

• PR Quality Gate, judges PR quality not authorship, lets competent bots through, blocks lazy humans
• anti-slop, 34 configurable checks, built from maintaining Coolify (120 slop PRs/month)
• agentscan-action, deep author profile analysis, detects automation patterns from account activity

Full writeup