fix(ci): preserve existing Kache config

[jmagar]

Summary

• preserve any existing non-empty Kache config on persistent/self-hosted runners
• continue generating filesystem or local-only config on clean runner homes
• prevent CI jobs from stripping dookie's NFS remote and daemon settings

Incident

A temporary self-hosted runner on dookie rewrote ~/.config/kache/config.toml to a 40 GiB local-only file because /home/runner/kache-remote was absent. This guard keeps host-owned configuration intact.

Validation

• existing config path: preserved
• empty runner home with writable farm remote: generated filesystem config
• empty runner home without remote: generated local-only fallback