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/send-user-message.ts

@@ -0,0 +1,97 @@
+/**
+ * Send User Message Example
+ *
+ * Demonstrates pi.sendUserMessage() for sending user messages from extensions.
+ * Unlike pi.sendMessage() which sends custom messages, sendUserMessage() sends
+ * actual user messages that appear in the conversation as if typed by the user.
+ *
+ * Usage:
+ *   /ask What is 2+2?     - Sends a user message (always triggers a turn)
+ *   /steer Focus on X     - Sends while streaming with steer delivery
+ *   /followup And then?   - Sends while streaming with followUp delivery
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	// Simple command that sends a user message
+	pi.registerCommand("ask", {
+		description: "Send a user message to the agent",
+		handler: async (args, ctx) => {
+			if (!args.trim()) {
+				ctx.ui.notify("Usage: /ask <message>", "warning");
+				return;
+			}
+
+			// sendUserMessage always triggers a turn when not streaming
+			// If streaming, it will throw (no deliverAs specified)
+			if (!ctx.isIdle()) {
+				ctx.ui.notify("Agent is busy. Use /steer or /followup instead.", "warning");
+				return;
+			}
+
+			pi.sendUserMessage(args);
+		},
+	});
+
+	// Command that steers the agent mid-conversation
+	pi.registerCommand("steer", {
+		description: "Send a steering message (interrupts current processing)",
+		handler: async (args, ctx) => {
+			if (!args.trim()) {
+				ctx.ui.notify("Usage: /steer <message>", "warning");
+				return;
+			}
+
+			if (ctx.isIdle()) {
+				// Not streaming, just send normally
+				pi.sendUserMessage(args);
+			} else {
+				// Streaming - use steer to interrupt
+				pi.sendUserMessage(args, { deliverAs: "steer" });
+			}
+		},
+	});
+
+	// Command that queues a follow-up message
+	pi.registerCommand("followup", {
+		description: "Queue a follow-up message (waits for current processing)",
+		handler: async (args, ctx) => {
+			if (!args.trim()) {
+				ctx.ui.notify("Usage: /followup <message>", "warning");
+				return;
+			}
+
+			if (ctx.isIdle()) {
+				// Not streaming, just send normally
+				pi.sendUserMessage(args);
+			} else {
+				// Streaming - queue as follow-up
+				pi.sendUserMessage(args, { deliverAs: "followUp" });
+				ctx.ui.notify("Follow-up queued", "info");
+			}
+		},
+	});
+
+	// Example with content array (text + images would go here)
+	pi.registerCommand("askwith", {
+		description: "Send a user message with structured content",
+		handler: async (args, ctx) => {
+			if (!args.trim()) {
+				ctx.ui.notify("Usage: /askwith <message>", "warning");
+				return;
+			}
+
+			if (!ctx.isIdle()) {
+				ctx.ui.notify("Agent is busy", "warning");
+				return;
+			}
+
+			// sendUserMessage accepts string or (TextContent | ImageContent)[]
+			pi.sendUserMessage([
+				{ type: "text", text: `User request: ${args}` },
+				{ type: "text", text: "Please respond concisely." },
+			]);
+		},
+	});
+}