fix(mom): use coding-agent SessionManager instead of custom MomSessionManager

Removes MomSessionManager which was missing methods expected by AgentSession
(appendMessage, getBranch, etc.), causing runtime crashes.

Now uses the standard SessionManager from coding-agent with a fixed context.jsonl
path per channel. The syncLogToSessionManager function handles syncing messages
from log.jsonl using SessionManager's public API.

Ref #595

packages/mom/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Use coding-agent's SessionManager instead of custom MomSessionManager to fix API mismatch crash ([#595](https://github.com/badlogic/pi-mono/issues/595))
+
 ## [0.42.4] - 2026-01-10
 
 ## [0.42.3] - 2026-01-10