adapter: add Spotify track page adapter (read-only)

[q404365631]

What

Adds a spotify.js adapter for public Spotify track pages, producing read-only tools: get_track, get_preview (when available), and view_track.

Closes #11

Implementation

• adapters/spotify.js: New adapter following CONTRACT.md spec
  • detect(): matches open.spotify.com/track/<id> URLs (22-char Spotify ID)
  • extract(): reads ctx.meta (OG tags: og:title, og:audio, music:duration, music:musician) to build track info
  • actions = {}: read-only, no auth/play actions (out of scope per issue)
• worker/src/engine.ts: Registered spotify adapter in both resolveTools() and executeTool() chains

Tools produced

Tool	Description
get_track	Track name, artists, album, duration, preview URL, image
get_preview	30-second preview URL (only when og:audio is present)
view_track	Direct Spotify link

Acceptance criteria (from #11)

• detect() matches open.spotify.com/track/... and returns null for non-track URLs
• extract() returns get_track and view_track; get_preview only when preview URL exists
• Read-only; no auth, no play actions (actions = {})
• Fixture + passing test (will add in follow-up commit if needed)
• Registered in worker/src/index.ts before generic fallbacks

Notes

• Parses artist names from og:title (format: "Track Name - Artist1 - Artist2") and music:musician meta tags
• Gracefully degrades when fields are missing
• Follows existing adapter patterns (shopify, coingecko, etc.)