Fix SessionEntry type to exclude SessionHeader

- SessionEntry now only contains conversation entries (messages, compaction, etc.)
- SessionHeader is separate, not part of SessionEntry
- FileEntry = SessionHeader | SessionEntry (for file storage)
- getEntries() filters out header, returns SessionEntry[]
- Added getHeader() for accessing session metadata
- Updated compaction and tests to not expect header in entries
- Updated mom package to use FileEntry for internal storage

packages/coding-agent/src/core/agent-session.ts

@@ -759,7 +759,7 @@ export class AgentSession {
 				if (lastEntry?.type === "compaction") {
 					throw new Error("Already compacted");
 				}
-				throw new Error("Nothing to compact (session too small or needs migration)");
+				throw new Error("Nothing to compact (session too small)");
 			}
 
 			// Find previous compaction summary if any