Librarian: answer each query back to the bot that sent it

So one librarian can serve several bots (test + deploy) instead of firing
every result/pong at a single static CONJURER_MAIN_BOT.

* The bot includes its own callback address (CONJURER_SELF_CALLBACK) in
  every /query and /ping.
* The librarian stores that callback with the query (persisted with the
  request, so a replay after restart still answers the right bot) and, for
  results, in the OUTBOX entry ({target, payload}) so the resender delivers
  to the origin bot even across a librarian restart.
* Pongs go back to the pinging bot too - otherwise a second bot's health
  check would be ponged to the first and always time out, so it could
  never enable its librarian cog.
* Empty callback falls back to MAIN_BOT_ADDRESS, and a legacy OUTBOX entry
  (raw payload, pre-callback) is still delivered to the default bot, so the
  upgrade is seamless.

Tests: per-origin result delivery + legacy-shape fallback (outbox),
busy/idle pong routed to the callback bot vs default (lifecycle). Suite:
58 unit + 52 integration green.

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

bot.py

@@ -39,6 +39,7 @@ from constants import (
     LIBRARIAN_SERVICE_ADDRESS,
     LOGFILE,
     RADIO_SERVICE_ADDRESS,
+    SELF_CALLBACK,
     TOKEN,
     service_headers,
 )
@@ -173,6 +174,7 @@ async def _load_service_groups() -> bool:
                 LIBRARIAN_PING,
                 service_headers(),
                 LIBRARIAN_PING_TIMEOUT,
+                SELF_CALLBACK,
             )
             if not alive:
                 logger.warning(