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/docs/extensions.md

@@ -632,7 +632,7 @@ pi.registerTool({
 
 ### pi.sendMessage(message, options?)
 
-Inject a message into the session:
+Inject a custom message into the session:
 
 ```typescript
 pi.sendMessage({
@@ -653,6 +653,34 @@ pi.sendMessage({
   - `"nextTurn"` - Queued for next user prompt. Does not interrupt or trigger anything.
 - `triggerTurn: true` - If agent is idle, trigger an LLM response immediately. Only applies to `"steer"` and `"followUp"` modes (ignored for `"nextTurn"`).
 
+### pi.sendUserMessage(content, options?)
+
+Send a user message to the agent. Unlike `sendMessage()` which sends custom messages, this sends an actual user message that appears as if typed by the user. Always triggers a turn.
+
+```typescript
+// Simple text message
+pi.sendUserMessage("What is 2+2?");
+
+// With content array (text + images)
+pi.sendUserMessage([
+  { type: "text", text: "Describe this image:" },
+  { type: "image", source: { type: "base64", mediaType: "image/png", data: "..." } },
+]);
+
+// During streaming - must specify delivery mode
+pi.sendUserMessage("Focus on error handling", { deliverAs: "steer" });
+pi.sendUserMessage("And then summarize", { deliverAs: "followUp" });
+```
+
+**Options:**
+- `deliverAs` - Required when agent is streaming:
+  - `"steer"` - Interrupts after current tool, remaining tools skipped
+  - `"followUp"` - Waits for agent to finish all tools
+
+When not streaming, the message is sent immediately and triggers a new turn. When streaming without `deliverAs`, throws an error.
+
+See [send-user-message.ts](../examples/extensions/send-user-message.ts) for a complete example.
+
 ### pi.appendEntry(customType, data?)
 
 Persist extension state (does NOT participate in LLM context):