docs: add music discovery, autoplay, and Mumble admin sections

- USAGE.md: music discovery (!similar, !tags), autoplay discovery
  config, Mumble admin (!mu) command reference
- CHEATSHEET.md: music discovery and Mumble admin quick reference
- ROADMAP.md: mark v2.4.0 as done, add MB fallback + !mu + autoplay
- TODO.md: mark music discovery and performance items as done
- PROJECT.md: update plugin categories table
- TASKS.md: close open doc items

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ROADMAP.md

@@ -179,11 +179,14 @@
 - [x] Alias plugin (!alias add/del/list)
 - [x] Container management tools (tools/build, start, stop, restart, nuke, logs, status)
 
-## v2.4.0 -- Music Discovery + Performance
+## v2.4.0 -- Music Discovery + Performance (done)
 
-- [ ] Last.fm integration (artist.getSimilar, artist.getTopTags, track.getSimilar)
-- [ ] `!similar` command (find similar artists, optionally queue via YouTube)
-- [ ] `!tags` command (genre/style tags for current track)
+- [x] Last.fm integration (artist.getSimilar, artist.getTopTags, track.getSimilar)
+- [x] `!similar` command (find similar artists, optionally queue via YouTube)
+- [x] `!tags` command (genre/style tags for current track)
+- [x] MusicBrainz fallback for `!similar` and `!tags` (no API key required)
+- [x] Auto-discover similar tracks during autoplay via Last.fm/MusicBrainz
+- [x] Mumble server admin plugin (`!mu` -- kick, ban, mute, move, channels)
 - [x] Pause/unpause (`!pause` toggle, position tracking, stale re-download)
 - [x] Autoplay continuous radio (random kept, silence-aware, cooldown between tracks)
 - [x] Periodic resume persistence (10s interval, survives hard kills)