detect: exclude fixture dirs by default

[pengfei-threemoonslab]

Summary

• skip common fixture corpus directories during canonical detect candidate discovery
• mirror the behavior in the zero-install detector and bump SCRIPT_VERSION to 0.2.1
• document the default skip behavior and add canonical/zero-install regressions
• update audited trust-model line pins after the detector skip-set change moved existing git probe call sites
• refresh llms-full.txt after changing docs/agent-recipes.md

Verification

• pytest tests/test_adapter_static_only.py tests/test_detect.py tests/test_zero_install_detector.py tests/test_public_surface_contract.py::test_llms_full_is_up_to_date -> 445 passed, 1 skipped
• ruff check src/agents_shipgate/cli/discovery/artifacts.py tools/shipgate-detect.py tests/test_detect.py tests/test_zero_install_detector.py tests/test_adapter_static_only.py -> passed
• git diff --check -> passed