feat(so101_curobo): default cuRobo/URDF path to the auto-downloaded SO-101 cache URDF

[yinsong1986]

What

Default the SO-101 cuRobo / URDF-sim path to the auto-downloaded
strands-robots SO-101 cache URDF, so --planner curobo works
out-of-the-box on any box that already ran the MuJoCo demo (or has
internet) — no --curobo-urdf flag needed.

Why

examples/so101_curobo ships no assets of its own (pure Python +
docs). The default MuJoCo demo auto-downloads the SO-101 model into the
external ~/.strands_robots/assets/robotstudio_so101/ cache via
strands_robots.assets, and that download already includes a URDF
(so101_new_calib.urdf) and an assets/ mesh dir next to the MJCF.

But the cuRobo planner (planner.py:357) and the URDF-sim path
(controller.py:154) only looked at an explicit urdf_path /
SO101_URDF, so they made the user hand-stage a URDF even though a
perfectly good one was already on disk — and make_planner(prefer="auto")
silently fell back to the scripted planner whenever no URDF flag was set,
even with cuRobo installed.

How

New shared resolvers in planner.py with the documented precedence:

1. explicit urdf_path / asset_path argument,
2. SO101_URDF / SO101_ASSET env var,
3. the auto-resolved cache URDF + mesh dir
   (resolve_model_dir("so101"), which triggers the same auto-download
   the MuJoCo path uses).

resolve_so101_urdf(urdf_path=None)   # -> str | None
resolve_so101_asset(asset_path="")   # -> str  ("" when none resolve)

Wired into:

• CuroboMotionPlanner.__init__ (urdf_path / asset_path).
• _curobo_usable() — make_planner(prefer="auto") now selects cuRobo
  out-of-the-box when it's installed and the cache URDF exists.
• controller.py sim-arm URDF resolution — the sim loads the exact
  URDF cuRobo plans with (identical joint conventions + EE frame).
• plan_curobo_offline.py + app.py CLI help text + README.

Returns None when none of the three resolve, so callers keep their
existing fail-with-hint behaviour on a box with neither a flag nor the
cache. No change to the default MuJoCo + scripted path.

Verification

On this box the resolver now finds the cached asset with no flags:

resolve_so101_urdf(None)  -> ~/.strands_robots/assets/robotstudio_so101/so101_new_calib.urdf
resolve_so101_asset("")   -> ~/.strands_robots/assets/robotstudio_so101/assets

• pytest examples/so101_curobo/smoke_test.py — 4 passed: the existing
  MuJoCo pick-place smoke + the fix(so101): app.py / record_dataset fails on re-run — FileExistsError on default dataset dir (no overwrite/unique path) #143 re-run test (default path unchanged),
  plus two new tests:
  • test_resolve_so101_urdf_precedence (explicit → env → cache, mocked).
  • test_resolve_so101_urdf_none_when_unavailable.
• Lint: black --check, isort --check-only, flake8 all clean.

Notes

• This is the usability papercut flagged while auditing whether
  examples/so101_curobo is self-contained: it ships no assets, relies on
  the strands-robots auto-download cache for the default run, and now
  reuses that same cache for the cuRobo URDF instead of making the user
  stage one separately.
• Out of scope: the Isaac SO-101 USD path (T2) still needs a faithful
  USD; this PR only addresses the URDF used by cuRobo + the URDF-sim arm.

[cagataycali]

Approve. This removes a real usability papercut - the example ships no assets of its own yet forced a --curobo-urdf/SO101_URDF flag even though the MuJoCo path already auto-downloads a perfectly good URDF into the cache.

The resolve_so101_urdf / resolve_so101_asset precedence (explicit kwarg -> env -> cache) is well-factored, the fallback _so101_cache_urdf degrades safely (returns (None, "") on any import/resolve failure so callers keep their fail-with-hint behaviour), and wiring controller.py through the same resolver guarantees the sim arm loads the EXACT URDF cuRobo plans against - that shared-resolver invariant is the part that matters most for correctness.

One behavioural note worth flagging for the record (not blocking): _curobo_usable now routes through resolve_so101_urdf, so on a cache miss make_planner(prefer="auto") can trigger resolve_model_dir("so101")'s auto-download as a side effect of planner selection. That is the intended out-of-the-box behaviour per the PR description and is the same download the MuJoCo path already performs, so it is acceptable for an example - just calling it out so future readers know selection can touch the network on a clean box.

Tests properly mock _so101_cache_urdf (no real network in CI) and pin all three precedence levels plus the None fallback. Lint clean, default MuJoCo+scripted path unchanged. Good to merge.