fix: address PR review feedback for cargo-deny integration

- quality_gate_strict.sh: fail hard when cargo-deny is not installed
  instead of silently skipping, and let set -e handle check failures
- deny.toml: remove empty [graph].targets so cargo-deny checks all
  platforms instead of only the runner's default target

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

deny.toml

@@ -1,6 +1,3 @@
-[graph]
-targets = []
-
 [advisories]
 unmaintained = "workspace"
 yanked = "deny"

scripts/ci/quality_gate_strict.sh

@@ -8,11 +8,11 @@ echo "==> clippy (all warnings)"
 cargo clippy --locked --all --benches --tests --examples --all-features -- -D warnings
 
 echo "==> cargo deny"
-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)"
+if ! command -v cargo-deny &>/dev/null; then
+    echo "ERROR: cargo-deny not installed (install with: cargo install cargo-deny)"
+    exit 1
 fi
+cargo deny check
 
 echo "==> tests"
 cargo test --locked