Add list subcommand to enumerate matching contexts

[davidnortonjr]

Summary

• Adds a list subcommand that prints all contexts from kubeconfig, one per line
• Respects the global --include and --exclude filters already supported by other subcommands
• Includes tests covering unfiltered listing, include/exclude filtering, and error when no contexts match

Test plan

• kubectl-x list prints all contexts
• kubectl-x list --include prod prints only prod contexts
• kubectl-x list --exclude dev omits dev contexts
• kubectl-x list --include nonexistent returns an error
• go test ./cmd/... passes