feat(coding-agent): expose streamingBehavior on InputEvent

Add streamingBehavior to InputEvent so extensions can distinguish
idle prompts from mid-stream steers and queued follow-ups.

- Add streamingBehavior field to InputEvent type
- Thread it through ExtensionRunner.emitInput() and AgentSession.prompt()
- Add streaming-aware input gate example with tests
- Document in extensions.md

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

@@ -984,6 +984,7 @@ export class AgentSession {
 					currentText,
 					currentImages,
 					options?.source ?? "interactive",
+					options?.streamingBehavior,
 				);
 				if (inputResult.action === "handled") {
 					preflightResult?.(true);