fix: use cargo-deny action in CI, improve quality gate script

- Use EmbarkStudios/cargo-deny-action@v2 instead of cargo install
  for faster CI execution
- Fix quality_gate_strict.sh to check for cargo-deny availability
  instead of suppressing stderr

Co-Authored-By: Claude Opus 4.6 <[email protected]>

.github/workflows/code_style.yml

@@ -22,10 +22,8 @@ jobs:
     steps:
     - name: Checkout repository
       uses: actions/checkout@v6
-    - name: Install cargo-deny
-      run: cargo install cargo-deny
     - name: Run cargo deny
-      run: cargo deny check
+      uses: EmbarkStudios/cargo-deny-action@v2
 
   clippy:
     name: Clippy (${{ matrix.name }})

scripts/ci/quality_gate_strict.sh

@@ -8,7 +8,11 @@ echo "==> clippy (all warnings)"
 cargo clippy --locked --all --benches --tests --examples --all-features -- -D warnings
 
 echo "==> cargo deny"
-cargo deny check 2>/dev/null || echo "WARN: cargo-deny not installed, skipping"
+if command -v cargo-deny &>/dev/null; then
+    cargo deny check
+else
+    echo "WARN: cargo-deny not installed, skipping (install with: cargo install cargo-deny)"
+fi
 
 echo "==> tests"
 cargo test --locked