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!"
 

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 ..
 }
 

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}"