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/CHEATSHEET.md

@@ -581,6 +581,7 @@ HTML stripped on receive, escaped on send. IRC-only commands are no-ops.
 !volume 75               # Set volume (0-100, default 50)
 !keep                    # Keep current file + save metadata
 !kept                    # List kept files with metadata
+!kept rm <id>            # Remove a single kept track
 !kept clear              # Delete all kept files + metadata
 !kept repair             # Re-download missing kept files
 !duck                    # Show ducking status