feat: add batch export for multiple benchmarks

[aaronlippold]

Summary

• Batch export: Export multiple benchmarks in a single command
• Output directory: New --output-dir option for organized batch exports
• Progress tracking: Shows [1/N] progress for each benchmark
• Error resilience: Continues on individual failures with summary
• Backward compatible: Single ID export works exactly as before
• README docs fix: All links now point to GitHub Pages

Usage

# Export multiple benchmarks
cis-bench export 23598 22605 18208 --format yaml

# Export to specific directory
cis-bench export 23598 22605 --format xccdf --output-dir ./stig_exports

# Mixed: IDs and files
cis-bench export 23598 local_benchmark.json --format yaml

Test plan

• 15 new TDD tests in tests/unit/test_batch_export.py
• All 1121 tests passing
• Backward compatibility tests for single ID export
• Error handling tests (missing benchmarks, all failures)
• Progress tracking tests
• Output directory creation tests

Changes

File	Change
src/cis_bench/cli/commands/export.py	Refactored to support multiple identifiers
tests/unit/test_batch_export.py	15 new TDD tests
docs/user-guide/commands-reference.md	Updated export command docs
README.md	Fixed all doc links to GitHub Pages