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/parser.py

@@ -22,4 +22,8 @@ def build_parser() -> argparse.ArgumentParser:
     summary_parser = subparsers.add_parser("summary", help="Show .gx file summary")
     summary_parser.add_argument("path", help=".gx file to summarize")
 
+    lain_parser = subparsers.add_parser("lain", help="Execute .gx through LAIN runtime")
+    lain_parser.add_argument("path", help=".gx file to execute")
+    lain_parser.add_argument("-m", "--mode", default="analyze", help="Cognitive mode (default: analyze)")
+
     return parser