feat: attribution footer, interactive playground, and config extension

[GodsBoy]

Summary

• Attribution footer: All AI-generated comments (issue triage, PR summaries, release notes, code reviews) now include a configurable "Powered by RepoKeeper" footer with optional "Try it live" playground link. Default-on, opt-out via attribution.enabled: false. Duplicate detection and inline review comments intentionally excluded.
• Interactive playground: New GET /playground serves a self-contained HTML page where visitors can paste an issue or PR diff and get instant AI output. POST /playground/preview with dedicated rate limiting (10 req/15 min, separate from webhook limiter). Input validation, error handling, 503 responses for AI provider issues.
• Config extension: New attribution config block with enabled (boolean, default true) and playgroundUrl (optional string). Not included in per-repo YAML overrides by design.
• Existing footer migration: Three hardcoded footer strings replaced with shared withAttribution() utility.

Test plan

• pnpm build passes (zero TS errors)
• pnpm test passes (131 tests, all green)
• New tests: attribution utility (5), playground routes (8), responder attribution (3), summariser attribution (3)
• Verified attribution disabled path suppresses footer
• Verified duplicate detection comment does NOT get footer
• Verified playground returns 400 for invalid input, 503 for AI errors