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/domain/domain.go

@@ -1,5 +1,5 @@
-// Package domain holds the core commitment types and validation, ported from
-// the original Rust implementation. These are pure data types with no I/O.
+// Package domain holds the core commitment types and validation. These are
+// pure data types with no I/O.
 package domain
 
 import (