Opt-in HTTP runner-client boundary for the runner-local runtime adapter
(RFC hermes-run-adapter-contract / #1925, Slice 4c/4d). Default-OFF:
without HERMES_WEBUI_RUNNER_BASE_URL the factory preserves the bounded
'not configured' path; when set, WebUI acts only as a JSON HTTP client +
SSE bridge for start/observe/status/controls without owning runner maps.
New api/runner_client.py + additive _runner_* helpers in api/routes.py;
no change to the live _run_agent_streaming legacy path.
Co-authored-by: AJV20 <AJV20@users.noreply.github.com>
