feat(#116): synchronous group command dispatch with multi-agent response collection

Add addGroupCommandWithReplies() to AgentRegistryService that sends commands
to all LIVE agents in a group and returns CompletableFuture per agent for
collecting replies. Update sendGroupCommand() and pushConfigToAgents() to
wait with a shared 10-second deadline, returning CommandGroupResponse with
per-agent status, timeouts, and overall success. Config update endpoint now
returns ConfigUpdateResponse wrapping both the saved config and push result.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/dto/CommandGroupResponse.java

@@ -0,0 +1,13 @@
+package com.cameleer3.server.app.dto;
+
+import java.util.List;
+
+public record CommandGroupResponse(
+    boolean success,
+    int total,
+    int responded,
+    List<AgentResponse> responses,
+    List<String> timedOut
+) {
+    public record AgentResponse(String agentId, String status, String message) {}
+}

cameleer3-server-app/src/main/java/com/cameleer3/server/app/dto/ConfigUpdateResponse.java

@@ -0,0 +1,8 @@
+package com.cameleer3.server.app.dto;
+
+import com.cameleer3.common.model.ApplicationConfig;
+
+public record ConfigUpdateResponse(
+    ApplicationConfig config,
+    CommandGroupResponse pushResult
+) {}