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/modes/rpc/rpc-mode.ts

@@ -196,6 +196,11 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 						sessionManager: session.sessionManager,
 						modelRegistry: session.modelRegistry,
 						model: session.model,
+						isIdle: () => !session.isStreaming,
+						hasQueuedMessages: () => session.queuedMessageCount > 0,
+						abort: () => {
+							session.abort();
+						},
 					},
 				);
 			} catch (_err) {