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/test/input-transform-streaming-example.test.ts

@@ -0,0 +1,84 @@
+import { describe, expect, it, vi } from "vitest";
+import inputTransformStreaming from "../examples/extensions/input-transform-streaming.ts";
+import type {
+	ExecResult,
+	ExtensionAPI,
+	ExtensionContext,
+	InputEvent,
+	InputEventResult,
+} from "../src/core/extensions/index.ts";
+
+type InputHandler = (event: InputEvent, ctx: ExtensionContext) => Promise<InputEventResult | undefined>;
+
+function setup(execResult: ExecResult) {
+	let handler: InputHandler | undefined;
+
+	const exec = vi.fn<ExtensionAPI["exec"]>().mockResolvedValue(execResult);
+
+	const api = {
+		on: (event: string, h: InputHandler) => {
+			if (event === "input") handler = h;
+		},
+		exec,
+	} as unknown as ExtensionAPI;
+
+	inputTransformStreaming(api);
+
+	const ctx = {} as ExtensionContext;
+
+	function emit(text: string, streamingBehavior?: "steer" | "followUp") {
+		return handler!({ type: "input", text, source: "interactive", streamingBehavior }, ctx);
+	}
+
+	return { emit, exec };
+}
+
+describe("input-transform-streaming example", () => {
+	const diffOutput = " src/index.ts | 5 ++---\n 1 file changed, 2 insertions(+), 3 deletions(-)";
+	const gitSuccess: ExecResult = { stdout: diffOutput, stderr: "", code: 0, killed: false };
+	const gitEmpty: ExecResult = { stdout: "", stderr: "", code: 0, killed: false };
+	const gitFail: ExecResult = { stdout: "", stderr: "not a git repo", code: 128, killed: false };
+
+	it("skips exec during steering", async () => {
+		const { emit, exec } = setup(gitSuccess);
+		const result = await emit("what changes did I make?", "steer");
+		expect(result).toEqual({ action: "continue" });
+		expect(exec).not.toHaveBeenCalled();
+	});
+
+	it("transforms when idle and text matches trigger", async () => {
+		const { emit, exec } = setup(gitSuccess);
+		const result = await emit("review my changes");
+		expect(exec).toHaveBeenCalledWith("git", ["diff", "--stat"]);
+		expect(result).toMatchObject({ action: "transform" });
+		const text = (result as { text: string }).text;
+		expect(text).toContain("review my changes");
+		expect(text).toContain("src/index.ts");
+	});
+
+	it("transforms when queued as follow-up", async () => {
+		const { emit, exec } = setup(gitSuccess);
+		const result = await emit("show me the diff", "followUp");
+		expect(exec).toHaveBeenCalled();
+		expect(result).toMatchObject({ action: "transform" });
+	});
+
+	it("continues when text does not match trigger", async () => {
+		const { emit, exec } = setup(gitSuccess);
+		const result = await emit("explain this function");
+		expect(result).toEqual({ action: "continue" });
+		expect(exec).not.toHaveBeenCalled();
+	});
+
+	it("continues when git diff is empty", async () => {
+		const { emit } = setup(gitEmpty);
+		const result = await emit("any changes?");
+		expect(result).toEqual({ action: "continue" });
+	});
+
+	it("continues when git fails", async () => {
+		const { emit } = setup(gitFail);
+		const result = await emit("show modified files");
+		expect(result).toEqual({ action: "continue" });
+	});
+});