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>

docs/CHEATSHEET.md

@@ -605,6 +605,38 @@ Skip/stop/prev/seek fade out smoothly (~0.8s); volume ramps over ~1s.
 Auto-resumes on reconnect if channel is silent (waits up to 60s for silence).
 Mumble-only: `!play` replies with error on other adapters, others silently no-op.
 
+## Music Discovery
+
+```
+!similar              # Similar to currently playing track
+!similar <artist>     # Similar artists to named artist
+!similar play         # Queue a random similar track
+!similar play <artist># Queue similar track for named artist
+!tags                 # Genre tags for current artist
+!tags <artist>        # Genre tags for named artist
+```
+
+Uses Last.fm when API key is set; falls back to MusicBrainz automatically.
+Config: `[lastfm] api_key` or `LASTFM_API_KEY` env var.
+
+## Mumble Admin (admin)
+
+```
+!mu kick <user> [reason]     # Kick user
+!mu ban <user> [reason]      # Ban user
+!mu mute <user>              # Server-mute
+!mu unmute <user>            # Remove server-mute
+!mu deafen <user>            # Server-deafen
+!mu undeafen <user>          # Remove server-deafen
+!mu move <user> <channel>    # Move user to channel
+!mu users                    # List connected users
+!mu channels                 # List channels
+!mu mkchan <name> [parent]   # Create channel
+!mu rmchan <name>            # Remove empty channel
+!mu rename <old> <new>       # Rename channel
+!mu desc <channel> <text>    # Set channel description
+```
+
 ## Plugin Template
 
 ```python