docs: add copilot-instructions.md and file-level comments

- Create .github/copilot-instructions.md with project coding standards,
  architecture overview, threading model, and key rules for AI sessions
- Add module description comments to app.cpp, rpc_client.cpp, rpc_worker.cpp,
  embedded_daemon.cpp, xmrig_manager.cpp, console_tab.cpp, settings.cpp
- Add ASCII connection state diagram to app_network.cpp
- Remove /.github/ from .gitignore so instructions file is tracked

src/rpc/rpc_client.cpp

@@ -1,6 +1,9 @@
 // DragonX Wallet - ImGui Edition
 // Copyright 2024-2026 The Hush Developers
 // Released under the GPLv3
+//
+// rpc_client.cpp — JSON-RPC client over HTTPS using libcurl.
+// All calls are blocking; run on RPCWorker threads, never on main thread.
 
 #include "rpc_client.h"
 #include "../config/version.h"