expand ~ in identity projects_dir at config load (#1711)
load_config read projects_dir verbatim, so an identities.toml stanza using a leading ~ (as the credential-less anonymous example documents) reached Lima as a mountPoint, which must be absolute, and reached on-disk vergil.toml lookups as a literal ~ that never resolves — silently dropping the repo's [vm.] overlay (incl. required nested=true). Expand ~ once at load so every consumer gets an absolute path; preserve empty as empty for the not-configured check. Refs #1705.