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>

run_g20_plan.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Isaac G20 实机命令桥
+# 默认订阅 /g20/cb_left_hand_control_cmd（与真机相同 0~255）
+# 发布     /sim/isaac/g20/left/joint_state
+# PLAN 名义弧度: --plan-nominal
+# 旧无前缀话题: --legacy-sdk
+# 跟踪自检: --track-test
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+ISAAC_DIR="${ISAAC_SIM_DIR:-$REPO_ROOT/_build/linux-x86_64/release}"
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+export ISAAC_SIM_DIR="$ISAAC_DIR"
+
+exec "$ISAAC_DIR/python.sh" "$SCRIPT_DIR/isaac_g20_plan_bridge.py" "$@"