Add LinkerHand Isaac Sim bridge and G20 assets.

Includes G20 PLAN/cmd_u8 bridge, joint mapping, dual-hand sim scripts,
and G20/L20/O6 URDF with USD payloads for Isaac Sim integration.

Co-authored-by: Cursor <cursoragent@cursor.com>

test_g20_stability.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python3
+"""Run built-in G20 stability test (headless)."""
+import subprocess
+import sys
+from pathlib import Path
+
+ISAAC = Path(__file__).resolve().parents[1] / "_build/linux-x86_64/release"
+SCRIPT = Path(__file__).resolve().parent / "linker_hand_dual_sim.py"
+
+cmd = [str(ISAAC / "python.sh"), str(SCRIPT), "--headless", "--stability-test", *sys.argv[1:]]
+print("Running:", " ".join(cmd))
+raise SystemExit(subprocess.call(cmd))