Add run gateway storage helper

[binrogithub]

Motivation

• Provide persistent storage for MCP run artifacts under runs/<run_id>/ so gateway code can record previews, apply plans, run metadata, evidence, and a report.
• Ensure sensitive information is not persisted by sanitizing payloads and applying a minimal evidence whitelist before storage.
• Expose simple entrypoints for other code to record runs via hc_run and a reproducible hc_run_demo utility.

Description

• Add package scaffold hc_agent and hc_agent/mcp and introduce hc_agent/mcp/run_gateway.py which contains the new helper and utilities.
• Implement _sanitize_payload and _whitelist_evidence plus a RunStore dataclass that creates runs/<run_id>/ and writes preview.json, apply.json, run.json, evidence.json, and report.md using sanitized content.
• Add hc_run to persist provided run artifacts and hc_run_demo to create a timestamped demo run and persist demo artifacts.

Testing

• No automated tests were run.

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Codex Task