fix(coding-agent): persist custom working message across loader recreation

Replace transient pendingWorkingMessage with a persistent workingMessage
field on InteractiveMode, matching the workingIndicatorOptions pattern.
New loaders now use this.workingMessage || this.defaultWorkingMessage
instead of always falling back to the default.

Also add working-message-test.ts extension example.

closes #3566

packages/coding-agent/examples/extensions/working-message-test.ts

@@ -0,0 +1,25 @@
+/**
+ * Working Message Persistence Test
+ *
+ * Sets a custom working message and indicator on session start so you can
+ * verify they survive across loader recreations (e.g. between agent turns).
+ *
+ * Usage:
+ *   pi --extension examples/extensions/working-message-test.ts
+ *
+ * Then send a few messages in interactive mode. The working message should
+ * stay "Working... (custom)" with a brown dot indicator every time the
+ * loader appears, not revert to the default gray "Working...".
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+const CUSTOM_MESSAGE = "\x1b[38;2;155;86;63mWorking... (custom)\x1b[39m";
+const CUSTOM_INDICATOR = { frames: ["\x1b[38;2;155;86;63m●\x1b[39m"] };
+
+export default function (pi: ExtensionAPI) {
+	pi.on("session_start", async (_event, ctx) => {
+		ctx.ui.setWorkingMessage(CUSTOM_MESSAGE);
+		ctx.ui.setWorkingIndicator(CUSTOM_INDICATOR);
+	});
+}