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/core/extensions/index.ts

@@ -52,6 +52,7 @@ export type {
 	RegisteredCommand,
 	RegisteredTool,
 	SendMessageHandler,
+	SendUserMessageHandler,
 	SessionBeforeBranchEvent,
 	SessionBeforeBranchResult,
 	SessionBeforeCompactEvent,