refactor(CLAUDE.md): slim from 1549 to 318 lines via progressive disclosure

Move verbose sections to references/ files, keeping concise pointers in
CLAUDE.md. Zero content loss — all documentation preserved in reference
files that Claude loads on demand.

Moved to references/:
- plugin-architecture.md (296 lines) — architecture docs
- plugin-troubleshooting.md (441 lines) — installation debugging
- new-skill-guide.md (241 lines) — detailed templates/checklists
- promotion-policy.md (60 lines) — third-party request policy
- youtube-downloader/references/internal-sop.md — yt-dlp SOP

Also fixed: Available Skills #36-42 indentation, deduplicated 4x
versioning sections into one, removed stale notes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

youtube-downloader/references/internal-sop.md

@@ -0,0 +1,12 @@
+# YouTube Downloader Internal SOP
+
+Use this SOP to avoid common yt-dlp failures and confusion:
+
+1. Quote YouTube URLs in shell commands (zsh treats `?` as glob). Example: `'https://www.youtube.com/watch?v=VIDEO_ID'`.
+2. Ensure proxy is active for both yt-dlp and PO Token providers (HTTP_PROXY/HTTPS_PROXY/ALL_PROXY).
+3. If you see "Sign in to confirm you're not a bot", request cookie permission and use browser cookies.
+4. Start the PO Token provider before downloading. Prefer Docker bgutil; fall back to browser-based WPC when Docker is unavailable or fails.
+5. Use `web_safari` client when cookies are present; otherwise use `mweb` for PO tokens.
+6. Keep the browser window open while WPC is minting tokens and make sure it can reach YouTube through the same proxy.
+7. If you see "Only images are available" or "Requested format is not available", treat it as PO token failure and retry after fixing provider/browser state.
+8. If you see SSL EOF or fragment errors, treat it as proxy instability. Retry with progressive formats or switch to a more stable proxy.