Add write_secure_temp_file() helper that creates files with mode 0600
on Unix (owner read/write only) instead of the default 0644
(world-readable). All 41 temp kubeconfig write sites updated.
Kubeconfig files contain cluster credentials; world-readable temp files
would expose them to any local user on the system.
- Add detect_auth_method() to identify kubeconfig credential type
(exec plugin, bearer token, inline cert, file-path cert, basic auth)
and surface warnings when the auth requires an external binary or file
- Split test into Stage 1 (kubectl get --raw=/healthz, no auth) and
Stage 2 (kubectl cluster-info, authenticated), so connectivity and
auth failures are reported distinctly rather than collapsing both
into opaque memcache.go noise
- Output now includes auth method and per-stage result for faster
diagnosis of 'server requires credentials' vs unreachable host
