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/rpc/rpc-mode.ts

@@ -229,6 +229,11 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 					output(error(undefined, "extension_send", e.message));
 				});
 			},
+			sendUserMessageHandler: (content, options) => {
+				session.sendUserMessage(content, options).catch((e) => {
+					output(error(undefined, "extension_send_user", e.message));
+				});
+			},
 			appendEntryHandler: (customType, data) => {
 				session.sessionManager.appendCustomEntry(customType, data);
 			},