fix(object_detector): resolve the detector model from utils config

[nelay04]

Fixes #157

Summary

_run_object_detection called get_llm(ctx, name="object_detector") without is_utils=True. Since object_detector is a utils node, the lookup always raised AttributeError, which was silently caught, so detection ran on the operator model instead of the configured utils.object_detector model (gemini-robotics-er-2-preview in the stock config).

Changes

• artemis/agents/object_detector/object_detector.py: pass is_utils=True, and log a warning when falling back to the operator model so the fallback is no longer silent.
• tests/unit/agents/test_object_detector_model_resolution.py: two tests
  • the detector requests name="object_detector", is_utils=True and uses that model
  • if resolution fails, it falls back to the operator model and logs a warning

Verification

• New tests fail on current main (2 failed) and pass with the fix. They mock get_llm, so no API key is needed.
• Real config resolution (no mocks), stock config/artemis.jsonc:
  • before: falls back to gemini-3.8-flash (operator)
  • after: gemini-robotics-er-2-preview
• Live check with a real Gemini key on tests/tools/inputs/screenshot.jpg: the request went to gemini-robotics-er-2-preview and returned a point for the date input field of [428, 196] (0-1000 scale), matching its on-screen position (~[422, 196]).
• Full suite (Windows, Python 3.13): same pre-existing failure before and after (test_awake_service.py::test_discovery_keeps_only_pool_claimed_devices), with passed count 2186 → 2188 (the two new tests).
• ruff format --check, ruff check (changed files), scripts/quality_ratchet.py (754 / 0 / 18, unchanged), and pyright --project pyright-core.json are clean.

Note

Other utils nodes (hopper, outputter) already pass is_utils=True; this was the only call missing it.