feat: add ctx.ui.setWorkingMessage() extension API

Allows extensions to customize the streaming loader message.
Pass undefined to restore default.

packages/coding-agent/docs/extensions.md

@@ -1170,6 +1170,7 @@ Extensions can interact with users via `ctx.ui` methods and customize how messag
 - Async operations with cancel (BorderedLoader)
 - Settings toggles (SettingsList)
 - Status indicators (setStatus)
+- Working message during streaming (setWorkingMessage)
 - Widgets above editor (setWidget)
 - Custom footers (setFooter)
 
@@ -1256,6 +1257,10 @@ See [examples/extensions/timed-confirm.ts](../examples/extensions/timed-confirm.
 ctx.ui.setStatus("my-ext", "Processing...");
 ctx.ui.setStatus("my-ext", undefined);  // Clear
 
+// Working message (shown during streaming)
+ctx.ui.setWorkingMessage("Thinking deeply...");
+ctx.ui.setWorkingMessage();  // Restore default
+
 // Widget above editor (string array or factory function)
 ctx.ui.setWidget("my-widget", ["Line 1", "Line 2"]);
 ctx.ui.setWidget("my-widget", (tui, theme) => new Text(theme.fg("accent", "Custom"), 0, 0));