feat: add evidence index schema and validator module

[google-labs-jules]

Formalized the Evidence Index by adding a JSON schema, a Python validator module, tests, and documentation.

• Created case/evidence_index.schema.json defining the canonical schema.
• Created case/evidence_index.sample.json as an example.
• Created engine/agents/evidence_validator.py to cross-check CSV, sticky notes, and timeline inputs.
• Created tests/core/test_evidence_validator.py to test the validator.
• Created docs/EVIDENCE_INDEX.md to document the evidence index structure and usage.
• Added __init__.py to engine/ and engine/agents/ to make them importable packages.

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PR created automatically by Jules for task 5526859831085142734 started by @cyserman

[google-labs-jules]

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