chore: add cargo-machete workflow

Add cargo-machete CI workflow to detect unused Rust dependencies.

Co-authored-by: Codex <noreply@openai.com>

Author: codex

.github/workflows/cargo-machete.yml

@@ -0,0 +1,24 @@
+name: Cargo Machete
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 3 * * 1"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  detect-unused-dependencies:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-machete
+
+      - name: Run cargo machete
+        run: cargo machete .