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/test/compaction-extensions.test.ts

@@ -83,6 +83,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 			flagValues: new Map(),
 			shortcuts: new Map(),
 			setSendMessageHandler: () => {},
+			setSendUserMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
 			setGetActiveToolsHandler: () => {},
 			setGetAllToolsHandler: () => {},
@@ -111,6 +112,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 		extensionRunner.initialize({
 			getModel: () => session.model,
 			sendMessageHandler: async () => {},
+			sendUserMessageHandler: async () => {},
 			appendEntryHandler: async () => {},
 			getActiveToolsHandler: () => [],
 			getAllToolsHandler: () => [],
@@ -284,6 +286,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 			flagValues: new Map(),
 			shortcuts: new Map(),
 			setSendMessageHandler: () => {},
+			setSendUserMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
 			setGetActiveToolsHandler: () => {},
 			setGetAllToolsHandler: () => {},
@@ -339,6 +342,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 			flagValues: new Map(),
 			shortcuts: new Map(),
 			setSendMessageHandler: () => {},
+			setSendUserMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
 			setGetActiveToolsHandler: () => {},
 			setGetAllToolsHandler: () => {},
@@ -376,6 +380,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 			flagValues: new Map(),
 			shortcuts: new Map(),
 			setSendMessageHandler: () => {},
+			setSendUserMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
 			setGetActiveToolsHandler: () => {},
 			setGetAllToolsHandler: () => {},