sim/mujoco: MjSpec stage 6 - agent-authored raw MJCF (replace_scene_mjcf / patch_scene_mjcf) (#121)

[cagataycali]

Part of #121. See IDEA.md stage 6.

Goal

The new capability unlocked by the refactor: let agents write MJCF elements that SimObject can't express (<tendon>, <equality>, <pair>, custom friction/solref/solimp, sites, flex bodies, hfields, skins, plugins, etc).

API

Two new @tool-exposed actions:

def replace_scene_mjcf(world, xml: str):
    \"\"\"Atomically swap the whole scene to agent-written MJCF.
    Validated by actually compiling it. Raises on failure with the
    MuJoCo compiler error verbatim.\"\"\"
    new_spec = mujoco.MjSpec.from_string(xml)
    new_model = new_spec.compile()  # raises if invalid
    ...

def patch_scene_mjcf(world, ops: list[dict]):
    \"\"\"Apply a list of structured ops to the live spec.
    ops = [
      {'op': 'add_body', 'parent': 'world', 'name': 'foo', 'pos': [...]},
      {'op': 'add_geom', 'body': 'foo', 'type': 'box', ...},
      {'op': 'set_attr', 'path': 'body/foo', 'attr': 'pos', 'value': [1,0,0]},
      {'op': 'delete',   'path': 'body/foo'},
    ]\"\"\"

Acceptance

• Both tool actions in tool_spec.json enum.
• Wrapped spec.compile() in try/except with clear error-shape parity with the rest of the tool surface.
• Integration test: agent writes a scene with a <tendon> element, confirms it compiles and simulates.
• Documented in the main tool docstring as an 'escape hatch' from the SimObject vocabulary.

[cagataycali]

Delivered in PR #85.

Commits:

• ad1d298 feat(sim/mujoco): MjSpec-based builder (stages 0-2 of IDEA.md)
• c2e8826 feat(sim/mujoco): finish MjSpec refactor (stages 3-7 of IDEA.md)
• 3404809 feat(sim/mujoco): add patch_scene_mjcf structured-op MJCF mutator

Verified: hatch run test tests/simulation/mujoco/ → 428 passed, 1 skipped. Lint clean.