chore: add benchmark artifacts to gitignore

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: doublegate

.gitignore

@@ -137,3 +137,7 @@ Desktop.ini
 
 # GitHub App credentials
 gha-creds-*.json
+
+# Benchmark artifacts
+benchmarks/*/Cargo.lock
+benchmarks/*/target/