Align Velociraptor action-space docs with raptor.xml and clarify tail actuation intent

[Copilot]

The Velociraptor MJCF model and docs were out of sync: docs described a 12D action space while the model/environment expose 15 actuators. This update tightens raptor.xml commentary and aligns README docs with the actual control interface.

• Model documentation clarity (environments/velociraptor/assets/raptor.xml)

  • Clarified tail design comments to explicitly state that distal tail segments are passive.
  • Clarified actuator comment that the tail actuator count is intentionally kept at 3 to preserve the 15D action space.

• Velociraptor docs accuracy (environments/velociraptor/README.md)

  • Updated Action Space from dim=12 to dim=15.
  • Extended actuator index table to include tail controls (12-14).

• Root docs consistency (README.md)

  • Updated Velociraptor action summary from leg+claw-only wording to include tail controls.

### Action Space (dim=15)

| Index | Actuator | Type |
|-------|----------|------|
| 0-4   | Right leg (hip pitch/roll, knee, ankle, toe) | Position |
| 5     | Right sickle claw | Motor |
| 6-10  | Left leg (hip pitch/roll, knee, ankle, toe) | Position |
| 11    | Left sickle claw | Motor |
| 12-14 | Tail (pitch/yaw/pitch) | Position |

Original prompt

Please review the raptor xml and see if you have any improvements

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