Extensions: add pi.sendUserMessage() for sending user messages

Adds sendUserMessage() to the extension API, allowing extensions to send
actual user messages (role: user) rather than custom messages. Unlike
sendMessage(), this always triggers a turn and behaves as if the user
typed the message.

- Add SendUserMessageHandler type and sendUserMessage() to ExtensionAPI
- Wire handler through loader, runner, and all modes
- Implement via prompt() with expandPromptTemplates: false
- Add send-user-message.ts example with /ask, /steer, /followup commands
- Document in extensions.md

fixes #483

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -442,6 +442,11 @@ export class InteractiveMode {
 						this.showError(`Extension sendMessage failed: ${err instanceof Error ? err.message : String(err)}`);
 					});
 			},
+			sendUserMessageHandler: (content, options) => {
+				this.session.sendUserMessage(content, options).catch((err) => {
+					this.showError(`Extension sendUserMessage failed: ${err instanceof Error ? err.message : String(err)}`);
+				});
+			},
 			appendEntryHandler: (customType, data) => {
 				this.sessionManager.appendCustomEntry(customType, data);
 			},