ci: add anti-slop PR quality workflow

[prihiri]

Add anti-slop workflow to detect low-quality/AI-generated PRs automatically.

This adds a GitHub Actions workflow that runs on PR open/reopen and uses the anti-slop action to detect and automatically close low-quality PRs.

Default configuration:

• max-failures: 4 (balanced approach)
• Blocks PRs from main/master branches
• Limits PR size (50 files, 10000 lines)
• Requires description, limits emojis/code references
• Detects spam usernames, requires 30-day account age
• Exempts owners/members/collaborators and known bots
• Closes PR when threshold exceeded

Closes: adds PR quality gate for the project.