feat: add SO-101 arm URDF with Gripper Forge compliant fingers

[zc-xzc]

Adds a complete SO-101 arm URDF whose fingers are the compliant TPU fingers exported by Gripper Forge, plus an injection script and an adaptation guide.

• so101_gripper_forge.urdf: full arm + gripper model; joints named to match SO-101 control conventions; left/right jaw meshes are real Gripper Forge geometry
• meshes/: example finger STLs (Universal preset, left + mirrored right)
• inject_gripper_forge.py: re-point any arm URDF at your own Gripper Forge finger STLs (auto-detect finger links, mirror the opposite jaw, mm->m scale); also supports --new to regenerate the bundled model
• README.md: usage plus a general method for adapting Gripper Forge to other two-jaw arms

Arm link dimensions in the bundled URDF are approximate placeholders; running the script on the official arm URDF keeps its exact link origins and swaps only the finger meshes.