Best-effort hook transport reachability

The live health of one hook's transport (GET /api/v1/admin/hooks/{name}/health). Checks whether the hook resolves to a LOADED kind: hook plugin in the process's plugin registry; this is a plugin-LOAD status check, not a network reachability probe: it never opens a connection, and it cannot tell you whether a kind: hook plugin's own configured external endpoint (e.g. busbar-webrequest-hook's settings.url) is actually reachable, only that the plugin itself is loaded. Never fires the hook. Additive-only.