fix: kept file protection, skip/autoplay, TTS routing, video ID expansion

- _cleanup_track: never delete files from kept directory (data/music/)
  even when track.keep=False -- fixes kept files vanishing on replay
- !kept rm: skip to next track if removing the currently playing one
- !skip: silent (no reply), restarts play loop for autoplay on empty queue
- TTS plays through merlin's own connection instead of derp's, preventing
  choppy audio when music and TTS compete for the same output buffer
- !play recognizes bare YouTube video IDs (11-char alphanumeric)
- !kept rm <id> subcommand for removing individual kept tracks

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

docs/USAGE.md

@@ -1628,7 +1628,7 @@ and voice transmission.
 !np                  Now playing
 !volume [0-100]      Get/set volume (persisted across restarts)
 !keep                Keep current track's audio file (with metadata)
-!kept [clear|repair] List kept files, clear all, or re-download missing
+!kept [rm <id>|clear|repair] List, remove, clear, or repair kept files
 !testtone            Play 3-second 440Hz test tone
 ```
 
@@ -1750,6 +1750,7 @@ file (natural dedup).
   duration) is fetched via yt-dlp and stored in `bot.state`
 - Use `!kept` to list preserved files with metadata (title, artist, duration,
   file size)
+- Use `!kept rm <id>` to remove a single kept track (file + metadata)
 - Use `!kept clear` to delete all preserved files and their metadata
 - Use `!kept repair` to re-download any kept tracks whose local files are
   missing (e.g. after a cleanup or volume mount issue)