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/examples/extensions/README.md

@@ -42,6 +42,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 | `qna.ts` | Extracts questions from last response into editor via `ctx.ui.setEditorText()` |
 | `status-line.ts` | Shows turn progress in footer via `ctx.ui.setStatus()` with themed colors |
 | `snake.ts` | Snake game with custom UI, keyboard handling, and session persistence |
+| `send-user-message.ts` | Demonstrates `pi.sendUserMessage()` for sending user messages from extensions |
 
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