Add YouTube Manager plugin

[spinnakergit]

Summary

Adds the YouTube Manager plugin to the marketplace index.

• Repo: https://github.com/spinnakergit/a0-youtube
• Tools: 8 (upload, search, read, comment, playlist, manage, analytics, transcript)
• Skills: 4 (upload, research, engage, manage)
• Auth: Google OAuth 2.0 with PKCE for full access, API key for public reads
• Security: CSRF-enforced endpoints, credential masking, 0o600 permissions on token files, error sanitization

Features

• Upload videos with title, description, tags, privacy, and thumbnails
• Search videos, channels, and playlists across YouTube
• Read video details, channel statistics, and subscription lists
• Manage comments: list, add, reply, delete
• Create and manage playlists
• Fetch video and channel analytics
• Extract video transcripts/captions

Testing

• Regression test suite (structural, no credentials required)
• Automated HV suite (API functional tests with credentials)
• Manual HV plan documented in tests/HUMAN_TEST_PLAN.md