Add trusted artifact boundary and publication

[dovvnloading]

Summary

• add the owner-bound trusted artifact boundary for source copy-in and provider output publication
• reject links/reparse points, hardlinks, sparse files, ADS/path ambiguity, active content, archives, and non-finite JSON
• add exact output claims, quarantine, hash/size limits, atomic publication, rollback, and fail-closed repository cleanup
• document the stage in the Phase 2 ADR/evidence and update the parent plan gates

Why

The signed bundle installer protects trusted runtime bytes but must not be the user-artifact path. This boundary prevents source mutation/overwrite, TOCTOU and path-link escapes, unclaimed outputs, active-content publication, and partial artifact batches before any provider is enabled.

Validation

• python -m pytest -q (187 passed, 1 skipped, 1 pre-existing warning)
• python -m pytest tests/test_phase2_artifact_boundary.py -q (16 passed)
• python -m compileall -q backend\\cortex_backend\\execution tests
• python tools/generate_contracts.py
• git diff --check
• frontend lint, typecheck, production build, and 39 frontend tests passed

The unrelated working-tree edit to Cortex.pyproj was intentionally not included.