feat(coding-agent): add input event for extension input interception (#761)

* feat(coding-agent): add input event for extension input interception

Extensions can now intercept, transform, or handle user input before the
agent processes it. Three result types: continue (pass through), transform
(modify text/images), handled (respond without LLM). Handlers chain
transforms and short-circuit on handled. Source field identifies origin.

* fix: make source public, use if/else over ternary

* fix: remove response field, extension handles own UI

packages/coding-agent/examples/extensions/input-transform.ts

@@ -0,0 +1,43 @@
+/**
+ * Input Transform Example - demonstrates the `input` event for intercepting user input.
+ *
+ * Start pi with this extension:
+ *   pi -e ./examples/extensions/input-transform.ts
+ *
+ * Then type these inside pi:
+ *   ?quick What is TypeScript?  → "Respond briefly: What is TypeScript?"
+ *   ping                        → "pong" (instant, no LLM)
+ *   time                        → current time (instant, no LLM)
+ */
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("input", async (event, ctx) => {
+		// Source-based logic: skip processing for extension-injected messages
+		if (event.source === "extension") {
+			return { action: "continue" };
+		}
+
+		// Transform: ?quick prefix for brief responses
+		if (event.text.startsWith("?quick ")) {
+			const query = event.text.slice(7).trim();
+			if (!query) {
+				ctx.ui.notify("Usage: ?quick <question>", "warning");
+				return { action: "handled" };
+			}
+			return { action: "transform", text: `Respond briefly in 1-2 sentences: ${query}` };
+		}
+
+		// Handle: instant responses without LLM (extension shows its own feedback)
+		if (event.text.toLowerCase() === "ping") {
+			ctx.ui.notify("pong", "info");
+			return { action: "handled" };
+		}
+		if (event.text.toLowerCase() === "time") {
+			ctx.ui.notify(new Date().toLocaleString(), "info");
+			return { action: "handled" };
+		}
+
+		return { action: "continue" };
+	});
+}