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/extensions/runner.ts

@@ -1036,7 +1036,12 @@ export class ExtensionRunner {
 	}
 
 	/** Emit input event. Transforms chain, "handled" short-circuits. */
-	async emitInput(text: string, images: ImageContent[] | undefined, source: InputSource): Promise<InputEventResult> {
+	async emitInput(
+		text: string,
+		images: ImageContent[] | undefined,
+		source: InputSource,
+		streamingBehavior?: "steer" | "followUp",
+	): Promise<InputEventResult> {
 		const ctx = this.createContext();
 		let currentText = text;
 		let currentImages = images;
@@ -1044,7 +1049,13 @@ export class ExtensionRunner {
 		for (const ext of this.extensions) {
 			for (const handler of ext.handlers.get("input") ?? []) {
 				try {
-					const event: InputEvent = { type: "input", text: currentText, images: currentImages, source };
+					const event: InputEvent = {
+						type: "input",
+						text: currentText,
+						images: currentImages,
+						source,
+						streamingBehavior,
+					};
 					const result = (await handler(event, ctx)) as InputEventResult | undefined;
 					if (result?.action === "handled") return result;
 					if (result?.action === "transform") {

packages/coding-agent/src/core/extensions/types.ts

@@ -756,6 +756,8 @@ export interface InputEvent {
 	images?: ImageContent[];
 	/** Where the input came from */
 	source: InputSource;
+	/** How the input will be delivered during streaming, or undefined when idle */
+	streamingBehavior?: "steer" | "followUp";
 }
 
 /** Result from input event handler */