Wire GX→LAIN runtime into CLI as 'lain' command

Add new command: gx lain <path.gx> [-m/--mode MODE]

Features:
- Execute .gx files through GX→LAIN runtime
- Display fused symbol, output text, diagnostics
- Configurable cognitive mode (default: analyze)
- Structured error reporting

Usage:
  gx lain sample_code.gx
  gx lain sample_code.gx -m synthesize

All integration tests still passing (18/18).

gx_cli/dispatcher.py

@@ -16,6 +16,8 @@ def dispatch(args) -> int:
         return commands.cmd_inspect(args.path)
     elif args.command == "summary":
         return commands.cmd_summary(args.path)
+    elif args.command == "lain":
+        return commands.cmd_lain(args.path, args.mode)
     else:
         print(f"Error: Unknown command: {args.command}", file=sys.stderr)
         return 1