checkRunnable() built the healthcheck spawn's minimalEnv.PATH from the
caller's PATH only, never merging in this Node's own bin dir the way
locateCommand's known-path search already does. npm-installed CLIs like
codex are `#!/usr/bin/env node` shebang scripts, so when the server is
launched with a minimal PATH (systemd/docker/PM2/Electron) lacking
node's dir, the healthcheck spawn fails even though the binary was
correctly located, and the tool shows as undetected.
Extracted the merge into a new buildHealthcheckPath() helper
(cliRuntimeHealthcheckPath.ts) to keep cliRuntime.ts within its frozen
file-size ceiling.
