Add hc_agent smoke tests and minimal helpers

[binrogithub]

Motivation

• Provide a small, testable hc-agent surface so smoke tests can validate preview/apply flows and run artifact wiring.
• Ensure preview code path never issues network calls and apply triggers exactly one outbound request.
• Verify gateway/runstore wiring writes run artifacts to runs/<run_id>/report.md so downstream tooling can consume reports.

Description

• Add hc_agent.py implementing preview(payload), apply(payload, request_fn=None), a RunStore helper that writes report.md, and gateway_run(...) which creates a run and writes a report.
• Add pytest smoke tests in tests/test_hc_agent_smoke.py that use monkeypatch/unittest.mock and a FakeResponse to exercise the three scenarios required: preview does not call requests.request, apply calls requests.request exactly once, and gateway run writes runs/<run_id>/report.md under a provided runstore root.
• Tests patch hc_agent.requests.request to avoid real network I/O and use tmp_path to isolate filesystem writes.

Testing

• No automated tests were executed as part of this change; the commit adds tests/test_hc_agent_smoke.py which is pytest-compatible and ready to run locally or in CI.

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Codex Task