feat: implement GNU-style command line options with pflag

- Add pflag dependency for POSIX/GNU-style command line parsing
- Replace Go standard flag package with pflag for better UX
- Implement long options with double dashes (--config, --max-messages, --help)
- Add short option aliases with single dashes (-c, -m, -h)
- Update help message with proper formatting and application description
- Update all documentation to reflect new flag syntax
- Update test scripts to use new command line format

GNU-style options provide better usability:
- Long descriptive options with --flag-name format
- Short single-character aliases for common options
- Standard help flag behavior with --help/-h
- Compatible with shell completion and standard conventions

Command line interface now supports:
- --config/-c FILE: Path to configuration file
- --max-messages/-m N: Message processing limit per mailbox
- --help/-h: Show help message and exit

All existing functionality preserved with improved command line experience.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

test/run-tests.sh

@@ -83,7 +83,7 @@ cd ../test
 
 # Run mail2couch with test configuration
 print_status "Running mail2couch with test configuration..."
-../go/mail2couch -config config-test.json -max-messages 3
+../go/mail2couch --config config-test.json --max-messages 3
 
 # Verify results
 print_status "Verifying test results..."
@@ -112,7 +112,7 @@ done
 
 # Test sync mode by running again (should show incremental behavior)
 print_status "Running mail2couch again to test incremental sync..."
-../go/mail2couch -config config-test.json -max-messages 3
+../go/mail2couch --config config-test.json --max-messages 3
 
 print_status "🎉 Basic integration tests completed successfully!"