[codex] Add e2e matrix coverage contract

[ErenAri]

Summary

• emit per-category coverage counters from the file-enforcement E2E matrix summary
• enforce required edge-case categories and category/top-level counter consistency in the summary validator
• align public evidence docs with the current workflow and note that older privileged evidence predates category coverage

Validation

• bash -n scripts/e2e_file_enforcement_matrix.sh
• python3 -m py_compile scripts/validate_e2e_matrix_summary.py
• python3 scripts/validate_e2e_matrix_summary.py --summary tests/fixtures/e2e_matrix_summary/full_coverage.json --min-total-checks 100 --max-failed-checks 0
• python3 scripts/validate_json_samples.py
• python3 tests/check_docs_security_contract.py
• python3 tests/check_guarantees_contract.py
• python3 tests/check_feature_surface_contract.py
• python3 tests/check_runbooks_contract.py docs/runbooks
• git diff --check