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/custom-tools/types.ts

@@ -47,6 +47,12 @@ export interface CustomToolContext {
 	modelRegistry: ModelRegistry;
 	/** Current model (may be undefined if no model is selected yet) */
 	model: Model<any> | undefined;
+	/** Whether the agent is idle (not streaming) */
+	isIdle(): boolean;
+	/** Whether there are queued messages waiting to be processed */
+	hasQueuedMessages(): boolean;
+	/** Abort the current agent operation (fire-and-forget, does not wait) */
+	abort(): void;
 }
 
 /** Session event passed to onSession callback */