Refactor session manager: migration chain, validation, tests

- Add migrateV1ToV2/migrateToCurrentVersion for extensible migrations
- createSummaryMessage now takes timestamp from entry
- loadEntriesFromFile validates session header
- findMostRecentSession only returns valid session files (reads first 512 bytes)
- Remove ConversationEntry alias
- Fix mom context.ts TreeNode type

Tests:
- migration.test.ts: v1 migration, idempotency
- build-context.test.ts: 14 tests covering trivial, compaction, branches
- file-operations.test.ts: loadEntriesFromFile, findMostRecentSession

packages/coding-agent/src/index.ts

@@ -112,8 +112,6 @@ export {
 	buildSessionContext,
 	type CompactionContent,
 	type CompactionEntry,
-	type ConversationContent,
-	type ConversationEntry,
 	CURRENT_SESSION_VERSION,
 	createSummaryMessage,
 	type FileEntry,
@@ -123,6 +121,7 @@ export {
 	type ModelChangeEntry,
 	migrateSessionEntries,
 	parseSessionEntries,
+	type SessionContent as ConversationContent,
 	type SessionContext as LoadedSession,
 	type SessionEntry,
 	type SessionHeader,