Add hc_agent MCP run gateway and storage helpers

[binrogithub]

Motivation

• Provide a simple MCP gateway to persist HC agent runs and return storage metadata for downstream consumption.
• Ensure run artifacts are consistently written to a runs/<run_id>/ layout with predictable filenames for preview, apply, evidence, run, and report content.
• Normalize and shrink payloads before persisting to avoid non-serializable values and oversized evidence blobs.

Description

• Add package scaffolding hc_agent/ and hc_agent/mcp/ and introduce hc_agent/mcp/run_gateway.py which defines RunRequest, RunResult, hc_run, and hc_run_demo helper functions.
• Add hc_agent/mcp/storage.py implementing sanitize_obj(...) to convert common Python types (dataclasses, Paths, Enums, bytes, collections) to JSON-serializable values.
• Implement build_min_evidence(...) to trim evidence dictionaries to a preferred set of keys when available and normalize lists/iterables.
• Implement init_run_dir(...) and write_run_artifacts(...) to create runs/<run_id>/ and write preview.json, apply.json, evidence.json, run.json, and report.md while applying sanitize_obj and build_min_evidence where appropriate.

Testing

• No automated tests were executed for this change.

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Codex Task