Add RunStore with atomic writes and index maintenance

[binrogithub]

Motivation

• Provide a centralized run artifact storage API that performs atomic writes to avoid partial/corrupt files.
• Maintain a lightweight runs/index.json metadata catalog for quick discovery and lifecycle operations containing {run_id, tenant_id, created_at, stage, ok}.
• Offer best-effort concurrency protection and lifecycle management via run deletion and garbage collection for stale runs.

Description

• Add hc_agent/core/run_store.py implementing RunStore with write_json, write_text, read_json, and read_text helpers that perform atomic writes using a temporary file and os.replace.
• Maintain runs/index.json in _update_index and _write_index_entries, ensuring entries include run_id, tenant_id, created_at, stage, and ok, with timestamps in ISO format and sorted by created_at.
• Implement delete_run(run_id) to remove a run directory and corresponding index entry, and gc(max_age_seconds) to remove runs older than a threshold and prune the index.
• Add a best-effort index lock (_index_lock) implemented via an exclusive lock file (.index.lock) to reduce race conditions while updating the index.

Testing

• No automated tests were run for this change.

---

Codex Task