Prepending cold-start thread backfill directly onto the message text
moved a recognized command (e.g. a bang-normalized "!queue ...") away
from character zero, so downstream command routing misclassified it as
conversational text and the command silently didn't run.
Route the backfill through MessageEvent.channel_context instead —
gateway.run already prepends channel_context after command dispatch
("[New message]" framing), so commands keep their COMMAND type while
the recovered history stays available to the agent.
Supersedes #68020, which dropped the fetched context entirely for
commands instead of preserving it out-of-band.
Salvaged from #66069 by @LevSky22.
fd433e046a