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/commands.py

@@ -4,6 +4,7 @@ from pathlib import Path
 from gx_compiler.compiler import GXCompiler, CompilerError
 from runtime_executor.integration import run_gx_with_summary
 from runtime_executor.gx_loader import load_gx, GXLoaderError
+from gx_lain.runtime import execute_gx_path as lain_execute_gx_path
 
 
 def cmd_compile(source_path: str, output_path: str = None) -> int:
@@ -129,3 +130,49 @@ def cmd_summary(gx_path: str) -> int:
     except Exception as e:
         print(f"Error: {e}", file=sys.stderr)
         return 1
+
+
+def cmd_lain(gx_path: str, mode: str = "analyze") -> int:
+    try:
+        gx = Path(gx_path)
+
+        if not gx.exists():
+            print(f"Error: File not found: {gx_path}", file=sys.stderr)
+            return 1
+
+        print(f"Executing {gx_path} through GX→LAIN runtime (mode: {mode})")
+
+        result = lain_execute_gx_path(
+            gx_path,
+            context={"cognitive_mode": mode}
+        )
+
+        # Print output text
+        print()
+        print(result["output_text"])
+        print()
+
+        # Print fused symbol
+        print("[Fused Symbol]")
+        print(f"  {result['fused_symbol']['summary']}")
+        if result['fused_symbol']['key_points']:
+            print(f"  Key points: {', '.join(result['fused_symbol']['key_points'])}")
+        print()
+
+        # Print diagnostics summary
+        print("[Diagnostics]")
+        elapsed = result['diagnostics']['elapsed']
+        print(f"  Elapsed: {elapsed:.4f}s")
+        print(f"  Interface: v{result['diagnostics']['interface_version']}")
+
+        if result['diagnostics']['errors']:
+            print(f"  Errors: {len(result['diagnostics']['errors'])}")
+            for err in result['diagnostics']['errors']:
+                print(f"    - {err['type']}: {err['message']}")
+            return 1
+
+        return 0
+
+    except Exception as e:
+        print(f"Error: {e}", file=sys.stderr)
+        return 1