feat(gateway): implement OpenResponses /v1/responses endpoint phase 2

- Add input_image and input_file support with SSRF protection
- Add client-side tools (Hosted Tools) support
- Add turn-based tool flow with function_call_output handling
- Export buildAgentPrompt for testing

src/commands/agent/types.ts

@@ -1,4 +1,5 @@
 import type { ChannelOutboundTargetMode } from "../../channels/plugins/types.js";
+import type { ClientToolDefinition } from "../../agents/pi-embedded-runner/run/params.js";
 
 /** Image content block for Claude API multimodal messages. */
 export type ImageContent = {
@@ -20,6 +21,8 @@ export type AgentCommandOpts = {
   message: string;
   /** Optional image attachments for multimodal messages. */
   images?: ImageContent[];
+  /** Optional client-provided tools (OpenResponses hosted tools). */
+  clientTools?: ClientToolDefinition[];
   /** Agent id override (must exist in config). */
   agentId?: string;
   to?: string;