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/mom/src/context.ts

@@ -98,9 +98,9 @@ export class MomSessionManager {
 		this.leafId = null;
 	}
 
-	private _createTreeNode(): TreeNode {
+	private _createTreeNode(): Omit<TreeNode, "type"> {
 		const id = uuidv4();
-		const node: TreeNode = {
+		const node = {
 			id,
 			parentId: this.leafId,
 			timestamp: new Date().toISOString(),