Implements --for-each flag for evaluating one CEL expression against
multiple context files separately, with comprehensive end-to-end test
coverage for all CLI features.

Features:
- New --for-each flag for batch context processing
- Evaluates expression separately for each file (not merged)
- Results displayed in table or JSON format with timing info
- Base context can be merged with each file context

Testing:
- Added 13 unit tests for batch context processing function
- Added 41 E2E tests using subprocess to verify full CLI experience
  - 25 tests for basic CLI features (context, output, flags, errors)
  - 7 tests for --file mode (expressions from files)
  - 9 tests for --for-each batch processing
- Total: 91 tests (50 unit + 41 E2E), all passing in ~7.5s
- CLI feature coverage: ~95% (excludes interactive/TUI modes)

Documentation:
- Updated README.md with batch processing example
- Added comprehensive --for-each docs to cli-reference.md
- Added usage recipes and patterns to cli-recipes.md
- Explained difference between --for-each (separate) vs --context-file (merged)

Example usage:
  cel 'user.age >= 18' --for-each user1.json --for-each user2.json --for-each user3.json
  cel 'expr' --context '{"base": 1}' --for-each file1.json --for-each file2.json --output json

The --for-each pattern with repeated flags makes it explicit that each
file is evaluated separately, following precedents from cargo test --test
and curl -H patterns.

Documents the CLI design patterns, batch processing rationale, E2E testing
strategy, and common pitfalls for future agents working on the codebase.

Key additions:
- CLI architecture overview (modes, key components)
- Batch processing design rationale (why --for-each pattern)
- E2E testing strategy and patterns
- Test organization (91 tests: 50 unit + 41 E2E)
- Common pitfalls for CLI development and testing
- Documentation requirements for new CLI features
- Performance notes

Replace custom OS detection logic with platformdirs library for proper
cross-platform configuration directory handling.

Changes:
- Add platformdirs>=4.0.0 to dependencies
- Update expression_storage.py to use platformdirs.user_config_dir()
- Fallback to manual detection if platformdirs not available
- Follows XDG Base Directory spec on Linux
- Uses proper AppData on Windows
- Uses Library/Application Support on macOS

Benefits:
- Proper handling of XDG_CONFIG_HOME on Linux
- Better Windows roaming profile support
- Follows platform conventions correctly
- Well-tested library (used by pip, setuptools, etc.)

… evaluation