The handleAutoCompactionStart handler was calling runBeforeCompaction with
only messageCount and an empty hook context. Plugins receiving this hook
could not identify the session or snapshot the transcript during
auto-compaction.
The other call site in compact.ts already passes the full payload
(messages, sessionFile, sessionKey). This aligns the subscribe handler
to do the same using ctx.params.session and ctx.params.sessionKey.
(cherry picked from commit 318a19d1a1)
