Token storage permission checks and path traversal mitigation

[pturbing]

Tokenstorage

mode bits before the umask and a permissive umask could leave them
writable by other local users. When the token directory already exists,
validate it rather than trusting it: reject symlinks and non-directories,
group/other-writable dirs, and dirs not owned by the current user, closing
a path-replacement vector on shared hosts.

Path Traversal

clusterURL.Path, which sanitizedURL then swaps into Host, bypassing the
non-empty-path check. The unvalidated host was used directly as a filename in
filepath.Join(f.dir, cluster), letting import/login/logout/export read, write,
or delete files outside the token directory.

Enforce containment at the store boundary: Load/Store/Delete now operate through
an os.Root rooted at the token directory, so the OS rejects any "..", absolute
path, or symlink escape. Add a tokenFileName guard that rejects keys that are
not a single clean path element, giving a clear error at the CLI boundary.
Legitimate hosts (with or without a port) are single path elements and pass
through unchanged, so existing token files remain readable.

Add regression tests covering the traversal vectors and filename validation.