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-incremental-sync.sh

@@ -67,7 +67,7 @@ build_app() {
 run_first_sync() {
     echo -e "\n${BLUE}Running first sync...${NC}"
     cd go
-    ./mail2couch -config ../test/config-test.json -max-messages 5
+    ./mail2couch --config ../test/config-test.json --max-messages 5
     cd ..
 }
 
@@ -138,7 +138,7 @@ EOF
 run_incremental_sync() {
     echo -e "\n${BLUE}Running incremental sync...${NC}"
     cd go
-    ./mail2couch -config ../test/config-test.json -max-messages 10
+    ./mail2couch --config ../test/config-test.json --max-messages 10
     cd ..
 }