feat: add provider-declared features system with Codex fast mode (#186)

Introduce a generic feature system where providers declare dynamic
features (toggles/selects) and the app renders controls automatically.
One message pair (set_agent_feature_request/response) handles all
feature mutations. Feature values persist and restore on agent resume.

First consumer: Codex fast mode (service_tier) — gated to supported
model families, with proper cleanup on model switch.

packages/server/src/client/daemon-client.ts

@@ -1707,6 +1707,35 @@ export class DaemonClient {
     }
   }
 
+  async setAgentFeature(agentId: string, featureId: string, value: unknown): Promise<void> {
+    const requestId = this.createRequestId();
+    const message = SessionInboundMessageSchema.parse({
+      type: "set_agent_feature_request",
+      agentId,
+      featureId,
+      value,
+      requestId,
+    });
+    const payload = await this.sendRequest({
+      requestId,
+      message,
+      timeout: 15000,
+      options: { skipQueue: true },
+      select: (msg) => {
+        if (msg.type !== "set_agent_feature_response") {
+          return null;
+        }
+        if (msg.payload.requestId !== requestId) {
+          return null;
+        }
+        return msg.payload;
+      },
+    });
+    if (!payload.accepted) {
+      throw new Error(payload.error ?? "setAgentFeature rejected");
+    }
+  }
+
   async setAgentThinkingOption(agentId: string, thinkingOptionId: string | null): Promise<void> {
     const requestId = this.createRequestId();
     const message = SessionInboundMessageSchema.parse({