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/docs/custom-tools.md

@@ -230,11 +230,33 @@ interface CustomToolContext {
   sessionManager: ReadonlySessionManager;  // Read-only access to session
   modelRegistry: ModelRegistry;            // For API key resolution
   model: Model | undefined;                // Current model (may be undefined)
+  isIdle(): boolean;                       // Whether agent is streaming
+  hasQueuedMessages(): boolean;            // Whether user has queued messages
+  abort(): void;                           // Abort current operation (fire-and-forget)
 }
 ```
 
 Use `ctx.sessionManager.getBranch()` to get entries on the current branch for state reconstruction.
 
+### Checking Queue State
+
+Interactive tools can skip prompts when the user has already queued a message:
+
+```typescript
+async execute(toolCallId, params, onUpdate, ctx, signal) {
+  // If user already queued a message, skip the interactive prompt
+  if (ctx.hasQueuedMessages()) {
+    return {
+      content: [{ type: "text", text: "Skipped - user has queued input" }],
+    };
+  }
+
+  // Otherwise, prompt for input
+  const answer = await pi.ui.input("What would you like to do?");
+  // ...
+}
+```
+
 ## Session Lifecycle
 
 Tools can implement `onSession` to react to session changes: