Add agent state methods to CustomToolContext and fix abort signature

CustomToolContext now has:
- isIdle() - check if agent is streaming
- hasQueuedMessages() - check if user has queued messages
- abort() - abort current operation (fire-and-forget)

Changed abort() signature from Promise<void> to void in both
HookContext and CustomToolContext. The abort is fire-and-forget:
it calls session.abort() without awaiting, so the abort signal
is set immediately while waitForIdle() runs in the background.

Fixes #388

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

@@ -1878,6 +1878,11 @@ export class AgentSession {
 			sessionManager: this.sessionManager,
 			modelRegistry: this._modelRegistry,
 			model: this.agent.state.model,
+			isIdle: () => !this.isStreaming,
+			hasQueuedMessages: () => this.queuedMessageCount > 0,
+			abort: () => {
+				this.abort();
+			},
 		};
 
 		for (const { tool } of this._customTools) {