Fix grouping for top-level static modules

[BananaAccurate]

Summary

• Fixes engine/antigravity_engine/hub/module_grouping.py so artifact detection is evaluated relative to the module root.
• Keeps real top-level source modules named static/ eligible for grouping while still excluding nested generated/static artifact directories.
• Adds a regression test covering a detected top-level static module with a nested generated static directory.

Why

A workspace can have a real top-level static/ source module. The previous artifact check evaluated absolute path parts, so any file under that module was skipped because static appeared in the full path. That caused refresh to load zero files for the module and produce no group agents.

Validation

• python -m pytest tests/test_hub_module_grouping.py -q passed: 9 passed.
• python -m pytest -q was also run; 219 passed and 5 failed for pre-existing environment/platform reasons unrelated to this patch: missing async pytest plugin handling for pytest.mark.asyncio tests and a Windows POSIX permission-mode assertion.
• Local package reinstall from engine/ with python -m pip install --user --no-deps --force-reinstall . succeeded.
• Installed-package regression check confirmed a top-level static/app.js loads while static/nested/static/generated.js remains excluded.

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[BananaAccurate]

Maintainer-ready closeout evidence for PR #91:

• Bug: real top-level static/ source modules were skipped as generated artifacts because artifact detection saw static in the absolute/module path and loaded zero source files for that module.
• Fix: evaluate artifact path parts relative to module_path, so top-level static/app.js loads while nested generated static/generated.js artifact paths remain excluded.
• Files changed: engine/antigravity_engine/hub/module_grouping.py and engine/tests/test_hub_module_grouping.py.
• Targeted validation: python -m pytest tests/test_hub_module_grouping.py -q -> 9 passed.
• Installed-package regression: static/app.js loads; nested static/generated.js remains excluded.
• JobApp smoke after local reinstall passed.
• Full suite note: 219 passed, 5 failed for unrelated environment/platform issues: missing async pytest plugin handling and a Windows POSIX permission-mode assertion.
• Maintainer action needed: upstream maintainer must review/merge; BananaAccurate has READ-only permission on study8677/antigravity-workspace-template and cannot merge this PR.