Session tree: simplify types, add branching API, comprehensive tests

Types:
- SessionEntryBase with type field, extended by all entry types
- CustomEntry for hooks (type: 'custom', customType, data)
- Remove XXXContent types and TreeNode (redundant)

API:
- Rename saveXXX to appendXXX with JSDoc explaining tree semantics
- Rename branchInPlace to branch() with better docs
- Add createBranchedSession(leafId) replacing index-based version
- Add getTree() returning SessionTreeNode[] for tree traversal
- Add appendCustomEntry(customType, data) for hooks

Tests:
- tree-traversal.test.ts: 28 tests covering append, getPath, getTree,
  branch, branchWithSummary, createBranchedSession
- save-entry.test.ts: custom entry integration

Docs:
- Class-level JSDoc explaining append-only tree model
- Method docs explaining leaf advancement and branching
- CHANGELOG.md entry for all changes

packages/mom/src/agent.ts

@@ -441,7 +441,7 @@ function createRunner(sandboxConfig: SandboxConfig, channelId: string, channelDi
 	});
 
 	// Load existing messages
-	const loadedSession = sessionManager.loadSession();
+	const loadedSession = sessionManager.buildSessionContex();
 	if (loadedSession.messages.length > 0) {
 		agent.replaceMessages(loadedSession.messages);
 		log.logInfo(`[${channelId}] Loaded ${loadedSession.messages.length} messages from context.jsonl`);
@@ -628,7 +628,7 @@ function createRunner(sandboxConfig: SandboxConfig, channelId: string, channelDi
 
 			// Reload messages from context.jsonl
 			// This picks up any messages synced from log.jsonl before this run
-			const reloadedSession = sessionManager.loadSession();
+			const reloadedSession = sessionManager.buildSessionContex();
 			if (reloadedSession.messages.length > 0) {
 				agent.replaceMessages(reloadedSession.messages);
 				log.logInfo(`[${channelId}] Reloaded ${reloadedSession.messages.length} messages from context`);