Add machine deletion API with uninstall command support

- Add AdminCommand message to protobuf (uninstall, restart, update)
- Add DELETE /api/machines/:agent_id endpoint with options:
  - ?uninstall=true - send uninstall command to online agent
  - ?export=true - return session history before deletion
- Add GET /api/machines/:agent_id/history endpoint for history export
- Add GET /api/machines endpoint to list all machines
- Handle AdminCommand in agent session handler
- Handle ADMIN_UNINSTALL error in agent main loop to trigger uninstall

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

proto/guruconnect.proto

@@ -278,6 +278,18 @@ message AgentStatus {
     bool is_streaming = 6;
 }
 
+// Server commands agent to uninstall itself
+message AdminCommand {
+    AdminCommandType command = 1;
+    string reason = 2;              // Why the command was issued
+}
+
+enum AdminCommandType {
+    ADMIN_UNINSTALL = 0;            // Uninstall agent and remove from startup
+    ADMIN_RESTART = 1;              // Restart the agent process
+    ADMIN_UPDATE = 2;               // Download and install update (future)
+}
+
 // ============================================================================
 // Top-Level Message Wrapper
 // ============================================================================
@@ -320,5 +332,8 @@ message Message {
 
         // Chat
         ChatMessage chat_message = 60;
+
+        // Admin commands (server -> agent)
+        AdminCommand admin_command = 70;
     }
 }