Land prototype on main (fix stacked-PR retarget gap)

PRs #10 and #11 were merged into their intermediate base branches
(restructure/working-copy-root and proto-improvements) rather than main,
because the stacked PRs' bases were not auto-retargeted (the branches were
not deleted on merge). As a result main only received the #9 restructure
and is still the plain working-copy bot.

This brings the full prototype onto main as a clean delta on top of the
current main tree (identical content to proto-improvements, but with main
ancestry so it merges without the squash-induced rename/delete conflicts):

- constants.py: env-var config, safe JSON loading, dependency guards,
  env->netrc tokens, API_SHARED_KEY + service_headers(), CONAN_* config
- communication_subroutine.py: queue timeout/Empty, daemon threads,
  cooperative stop_event, inbound _authorize_request()
- bot.py: asyncio event loop + load conanjurer_commands
- music_functions / radio_commands / librarian_commands: X-Conjurer-Api-Key
- conanjurer_commands/_functions: fixed + integrated bridge with RCON
  player-join notifications
- requirements_bot.txt: aiomcrcon, asyncssh
- conjurer_musician/.gitignore: keep runtime playlists/mp3 out of the repo

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

conjurer_musician/.gitignore

@@ -0,0 +1,7 @@
+# Runtime-generated radio data — keep out of the repo
+all_playlist.playlist
+hit.playlist
+request.playlist
+priority_queue.playlist
+prio_playlist.json
+*.mp3