feat(cli): add youtube and instagram commands

[DeanThompson]

Summary

Adds APIMux CLI and skill support for the new YouTube and Instagram social-data capabilities.

Motivation

Pairs with the APIMux service social-source MVP so users can run brand-monitoring workflows from the CLI: search videos/accounts/posts, inspect details, collect comments/replies, and use YouTube transcripts for summarization.

Changes

• Adds top-level apimux youtube ... commands for video search/detail/comments/replies, channel detail/videos, and transcripts.
• Adds top-level apimux instagram ... commands for search users/reels/hashtags, profile/posts/reels, post detail/comments/replies.
• Adds offline schema stubs and command tests for the new source commands.
• Adds skills/apimux-youtube and skills/apimux-instagram documentation.
• Follow-up alignment from self-test: YouTube search docs only claim first-wave query/cursor/count; youtube get_video_comments supports --url from live service schema.

Testing

• PASS: go test ./internal/command
• PASS: make test
• PASS: local service smoke against fake TikHub provider:
  • youtube search_videos --query 'Pimax VR' --count 5
  • youtube get_video_comments --url 'https://www.youtube.com/watch?v=dQw4w9WgXcQ' --count 5
  • instagram get_user_profile --username pimaxofficial
  • instagram get_post_comments --url 'https://www.instagram.com/p/CODE/' --count 5
• PASS: schema/help smoke confirms youtube get_video_comments --help exposes --url and --video-id from service schema.

Risk

Low to medium: CLI command exposure only, schema-bound to service capabilities; existing commands are unchanged.