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/index.ts

@@ -116,6 +116,7 @@ export {
 	type ConversationEntry,
 	CURRENT_SESSION_VERSION,
 	createSummaryMessage,
+	type FileEntry,
 	getLatestCompactionEntry,
 	type MessageContent,
 	type ModelChangeContent,