Add minimal evidence filtering and persist minimal evidence.json

[binrogithub]

Motivation

• Keep persisted evidence payloads small and predictable by filtering to a whitelisted set of keys before writing to disk.
• Prevent accidental leakage of extra metadata when gateway handlers write evidence.json.

Description

• Add hc_agent/core/evidence.py with build_min_evidence(evidence) which returns a dict containing only the allowed keys: service, endpoint, method, path, status, request_id, duration_ms, and signature_sha256.
• Add gateway/handlers.py with write_evidence_json(evidence, output_dir) which calls build_min_evidence() and writes the minimal payload to evidence.json using pretty-printed JSON.
• Add package initializer files hc_agent/__init__.py and hc_agent/core/__init__.py to expose the new module.

Testing

• No automated tests were run for this change.

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Codex Task