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");

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

@@ -17,7 +17,7 @@ import type { CompactionResult } from "../../core/compaction/index.js";
 
 export type RpcCommand =
 	// Prompting
-	| { id?: string; type: "prompt"; message: string; images?: ImageContent[] }
+	| { id?: string; type: "prompt"; message: string; images?: ImageContent[]; streamingBehavior?: "steer" | "followUp" }
 	| { id?: string; type: "steer"; message: string }
 	| { id?: string; type: "follow_up"; message: string }
 	| { id?: string; type: "abort" }