fix: don't check dotfiles repo state when URLs are empty

The diffDotfiles function was always checking the local ~/.dotfiles git
state, even when comparing snapshots with empty dotfiles URLs. This
caused test failures when the user's actual dotfiles repo had uncommitted
changes.

Fix: Only check dotfiles repo state if at least one URL is configured.
If both system and reference have empty dotfiles URLs, skip the git state
check entirely.

This makes the tests deterministic and not dependent on the user's local
dotfiles repo state.

Author: Jerry Xie

internal/diff/compare.go

@@ -171,6 +171,12 @@ func diffDotfiles(systemURL, referenceURL string) *DotfilesDiff {
 		dd.RepoChanged = &ValueChange{System: systemURL, Reference: referenceURL}
 	}
 
+	// Only check local dotfiles repo state if dotfiles are actually configured
+	// If both URLs are empty, there's no dotfiles setup to check
+	if sysNorm == "" && refNorm == "" {
+		return dd
+	}
+
 	// Check local dotfiles repo for dirty state
 	home, err := os.UserHomeDir()
 	if err != nil {