Purge leftover Rust implementation

Remove the legacy/ Rust tree (Cargo manifests, .rs sources, desktop
entry), drop Rust-specific .gitignore blocks and README note, and strip
now-dangling "ported from Rust" comments from the Go sources.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/evidence/evidence.go

@@ -30,8 +30,8 @@ type Source interface {
 	Watch(ctx context.Context, onChange func(WindowSnapshot))
 }
 
-// titleNoise matches the legacy clock/ratio/percent runs that pollute bucket
-// keys (e.g. "1:23", "50.5%", "-3.0"). Ported verbatim from the Rust impl.
+// titleNoise matches the clock/ratio/percent runs that pollute bucket
+// keys (e.g. "1:23", "50.5%", "-3.0").
 var titleNoise = regexp.MustCompile(`-?\d+([:.]\d+)+%?`)
 
 // extraSpace collapses the whitespace runs left behind by the scrubs below.