fix(kubectl): stop context names starting with "-" from being misread as a flag

[plttn]

The CLI-fallback path in _tide_item_kubectl.fish (used when $KUBECONFIG lists multiple files, the default config doesn't exist, or the file didn't parse) passes kubectl's output straight to string replace without --:

string replace -r '/(|default)$' '' $context

A context named e.g. -staging makes $context start with -, so string replace reads it as an unknown option instead of the string to operate on. Same bug as the one just fixed in _tide_pwd.fish (#98), just in a different file.

Add -- before $context, and a regression test for a context name starting with -.