feat(coding-agent): implement /tree command for session tree navigation

- Add TreeSelectorComponent with ASCII tree visualization
- Add AgentSession.navigateTree() for switching branches
- Add session_before_tree/session_tree hook events
- Add SessionManager.resetLeaf() for navigating to root
- Change leafId from string to string|null for consistency with parentId
- Support optional branch summarization when switching
- Update buildSessionContext() to handle null leafId
- Add /tree to slash commands in interactive mode

packages/coding-agent/src/core/custom-tools/types.ts

@@ -43,7 +43,7 @@ export interface SessionEvent {
 	/** Previous session file path, or undefined for "start" and "new" */
 	previousSessionFile: string | undefined;
 	/** Reason for the session event */
-	reason: "start" | "switch" | "branch" | "new";
+	reason: "start" | "switch" | "branch" | "new" | "tree";
 }
 
 /** Rendering options passed to renderResult */