Fix slash commands and hook commands during streaming

- Hook commands now execute immediately during streaming (they manage their own LLM interaction via pi.sendMessage())
- File-based slash commands are expanded and queued via steer/followUp during streaming
- prompt() accepts new streamingBehavior option ('steer' or 'followUp') for explicit queueing during streaming
- steer() and followUp() now expand file-based slash commands and error on hook commands
- RPC prompt command accepts optional streamingBehavior field
- Updated docs: rpc.md, sdk.md, CHANGELOG.md

fixes #420

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -244,10 +244,12 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 
 			case "prompt": {
 				// Don't await - events will stream
-				// Hook commands and file slash commands are handled in session.prompt()
+				// Hook commands are executed immediately, file slash commands are expanded
+				// If streaming and streamingBehavior specified, queues via steer/followUp
 				session
 					.prompt(command.message, {
 						images: command.images,
+						streamingBehavior: command.streamingBehavior,
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");