Add incremental cache controls to Bandit

Normalized profile

What this task asks for

Add incremental analysis caching with cache invalidation, import/export, pruning, and cache inspection CLI options. Unchanged files must return cached results. CLI must support --incremental/--no-incremental, --cache-dir, --cache-size-limit. Config file must support incremental_analysis.enabled, incremental_analysis.cache_directory, and incremental_analysis.cache_expiry_days. CLI must support --warm-cache to pre-populate cache without reporting issues (exit 0, results empty). --warm-cache implies --incremental mode. CLI must support --export-cache FILE to export cache to a JSON file. CLI must support --prune-cache DAYS to remove entries older than N days (exit 0). --cache-stats must include cache_file_size_bytes.