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/test-wildcard-patterns.sh

@@ -31,7 +31,7 @@ run_test() {
     
     # Run mail2couch and capture output
     cd go
-    if ./mail2couch -config "../test/$config_file" -max-messages "$max_messages" 2>&1; then
+    if ./mail2couch --config "../test/$config_file" --max-messages "$max_messages" 2>&1; then
         echo -e "${GREEN}✅ Test completed successfully${NC}"
     else
         echo -e "${RED}❌ Test failed${NC}"