Add hc_run_demo gateway flow

[binrogithub]

Motivation

• Provide a simple demo runner for gateway flows that walks through demo steps and persists artifacts for inspection and follow-up apply runs.
• Make it easy to run sequential groups (network, storage, compute) with per-step error handling and a consolidated summary/report for operators.

Description

• Add hc_agent package and hc_agent/core/run_gateway.py which implements hc_run_demo and helpers (_now_iso, _write_json, _render_report).
• hc_run_demo invokes the provided hc_run(group=..., mode=...) callable for steps (default network, storage, compute), records per-step payloads and stores them as steps/NN-<group>.<mode>.json under the given run_dir.
• On exceptions a step artifact records status: error, error, and traceback, the runner stops further steps, and overall status is set to error.
• The runner writes a consolidated run.json and a human-readable report.md which includes a suggested next apply command hc-run --mode apply --run-dir <run_dir>.

Testing

• No automated tests were executed for this change.

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Codex Task