fix: resolve ruff lint errors in benchmark suite

Remove unused imports (shutil, string, datetime, os, yaml, time,
SCALE_CONFIGS) and unused variable assignments in timing-only calls.

tests/benchmarks/test_ingest_bench.py

@@ -8,12 +8,10 @@ Measures mining performance at scale:
   - Re-ingest skip overhead (finding #11: file_already_mined check)
 """
 
-import os
 import time
 
 import chromadb
 import pytest
-import yaml
 
 from tests.benchmarks.data_generator import PalaceDataGenerator
 from tests.benchmarks.report import record_metric