add ambience music system: TUI crossfade, yt-dlp fetch, source tracking

- tools/run.py: pygame.mixer subsystem — polls session/ambience.md,
  crossfades tracks, shows ♫ in status bar
- tools/music-fetch.py: search/download from YouTube via yt-dlp,
  auto-increment filenames, --replace and --dry-run modes
- tools/ambience.py: companion CLI to set ambience state
- session/ambience.md: current ambience state file (DM writes here)
- session/ambience_options.md: ambience → file mapping table
- session/ambience_sources.md: file → YouTube URL tracking for re-download
- session/audio/ added to .gitignore (audio files not tracked in git)

.gitignore

@@ -1,3 +1,4 @@
 __pycache__/
 *.pyc
 .env
+session/audio/