feat(contract): add evidence pipeline schemas

[stacknil]

Summary

• add JSON Schema Draft 2020-12 contracts for TL-01 schema-contract outputs
• map rule_hits, case_bundles, dedup_explanations, investigation_signals, and investigation_summary schemas to committed demo artifacts
• add schema validation tests and link the evidence pipeline contract from README/reviewer docs

Validation

• python -m pytest tests/test_evidence_pipeline_schemas.py
• python -m pytest tests/test_evidence_pipeline_schemas.py tests/test_reviewer_docs.py tests/test_markdown_links.py
• python -m pytest --basetemp pytest-tmp-contract
• git diff --cached --check

Notes

• Full pytest required an elevated run because the managed sandbox denied access to pytest temporary directories.
• Schemas describe reviewer evidence artifacts only; no SIEM, dashboard, alert routing, autonomous response, or final verdict claims added.

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