feat(channels): support routed conversation compaction

[just-cameron]

Summary

• add admin-gated /compact handling across routed messaging channels and Slack mention commands
• target the exact bound agent conversation while reusing the existing compaction modes, hooks, reflection behavior, and error handling
• update channel help/docs and add route, argument, and authorization coverage
• extract compact/reload listener wiring from the oversized lifecycle module and lower its size ratchet

This fixes the recovery dead end where a channel conversation could become too large or prompt-blocked to process normal messages, while /compact itself was rejected by the channel command router.

Verification

• bun run check — 12/12 checks passed
• focused channel and listener suites — 222 tests passed
• bun run build — completed successfully
• bounded independent code review — no findings

AI Disclosure

• This pull request was written entirely by a human
• This pull request was written with AI assistance and reviewed and edited by a human
• I have read the AI Policy and agree to its terms

This draft was authored by Letta Code and is awaiting Cameron’s human review before it is marked ready.

AI Tool(s) Used

Letta Code implemented, tested, documented, and prepared the change. A separate Letta Code review agent performed a bounded read-only review and reported no findings.

Human Verification

Pending Cameron’s review.