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/statemachine/statemachine.go

@@ -1,5 +1,5 @@
 // Package statemachine holds the pure runtime and commitment transition
-// functions ported from the Rust implementation. No I/O, no shared state.
+// functions. No I/O, no shared state.
 package statemachine
 
 import (
@@ -9,8 +9,8 @@ import (
 )
 
 // RuntimeAction enumerates runtime transitions. Activate's policy acceptance
-// and admin-override exit targets from the Rust enum are flattened into
-// distinct actions so the action type stays a simple value.
+// and admin-override exit targets are flattened into distinct actions so the
+// action type stays a simple value.
 type RuntimeAction string
 
 const (