sim/mujoco: MjSpec stage 7 - remove feature flag, delete mjcf_builder.py (#121)

[cagataycali]

Part of #121. See IDEA.md stage 7.

Goal

Final cleanup. By this point (Stages 3-6 done) the SpecBuilder path is proven equivalent in CI.

Checklist

• Remove STRANDS_SIM_USE_MJSPEC env var and the Simulation._use_mjspec() helper.
• Delete strands_robots/simulation/mujoco/mjcf_builder.py.
• Audit scene_ops.py for any remaining dead helpers (_save_and_patch_xml, _patch_xml_paths, etc.) - target <500 LOC total.
• Remove any remaining if self._use_mjspec(): branches.
• Update AGENTS.md if scene-building conventions change.
• Update CHANGELOG.
• Verify grep -r "f'<" strands_robots/simulation/mujoco/ is empty.

Acceptance

• scene_ops.py < 500 LOC.
• No MJCFBuilder references anywhere in source or tests.
• CI green (same tests, just the spec path only).

[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.