Add session management and agent state methods to hooks API

HookAPI additions:
- pi.newSession(options?) - create new session with optional setup callback
- pi.branch(entryId) - branch from a specific entry
- pi.navigateTree(targetId, options?) - navigate the session tree

HookContext additions:
- ctx.isIdle() - check if agent is streaming
- ctx.waitForIdle() - wait for agent to finish
- ctx.abort() - abort current operation
- ctx.hasQueuedMessages() - check for queued user messages

These enable hooks to programmatically manage sessions (handoff, templates)
and check agent state before showing interactive UI.

Fixes #388

packages/coding-agent/src/core/agent-session.ts

@@ -558,6 +558,10 @@ export class AgentSession {
 			sessionManager: this.sessionManager,
 			modelRegistry: this._modelRegistry,
 			model: this.model,
+			isIdle: () => !this.isStreaming,
+			waitForIdle: () => this.agent.waitForIdle(),
+			abort: () => this.abort(),
+			hasQueuedMessages: () => this.queuedMessageCount > 0,
 		};
 
 		try {