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/examples/extensions/input-transform-streaming.ts

@@ -0,0 +1,39 @@
+/**
+ * Streaming-Aware Input Gate
+ *
+ * Demonstrates `event.streamingBehavior` to skip expensive pre-processing
+ * during mid-stream steering, where low latency matters.
+ *
+ * This extension prepends `git diff --stat` output when the user mentions
+ * file changes, giving the model immediate context. During steering the
+ * exec call is skipped so the correction reaches the model without delay.
+ *
+ * Start pi with this extension:
+ *   pi -e ./examples/extensions/input-transform-streaming.ts
+ */
+import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
+
+const TRIGGER = /\b(changes?|diff|modified)\b/i;
+
+export default function (pi: ExtensionAPI) {
+	pi.on("input", async (event) => {
+		// During steering, skip the exec call — corrections should be fast
+		if (event.streamingBehavior === "steer") {
+			return { action: "continue" };
+		}
+
+		if (!TRIGGER.test(event.text)) {
+			return { action: "continue" };
+		}
+
+		const { stdout, code } = await pi.exec("git", ["diff", "--stat"]);
+		if (code !== 0 || !stdout.trim()) {
+			return { action: "continue" };
+		}
+
+		return {
+			action: "transform",
+			text: `${event.text}\n\nCurrent uncommitted changes:\n\`\`\`\n${stdout.trim()}\n\`\`\``,
+		};
+	});
+}