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>

go/config/config.go

@@ -2,10 +2,11 @@ package config
 
 import (
 	"encoding/json"
-	"flag"
 	"fmt"
 	"os"
 	"path/filepath"
+
+	"github.com/spf13/pflag"
 )
 
 type Config struct {
@@ -87,23 +88,37 @@ type CommandLineArgs struct {
 	MaxMessages int
 }
 
-// ParseCommandLine parses command line arguments
+// ParseCommandLine parses command line arguments using GNU-style options
 func ParseCommandLine() *CommandLineArgs {
-	configFlag := flag.String("config", "", "Path to configuration file")
-	maxMessagesFlag := flag.Int("max-messages", 0, "Maximum number of messages to process per mailbox per run (0 = no limit)")
-	flag.Parse()
+	var args CommandLineArgs
 
-	return &CommandLineArgs{
-		ConfigPath:  *configFlag,
-		MaxMessages: *maxMessagesFlag,
+	// Define long options with -- and short options with -
+	pflag.StringVarP(&args.ConfigPath, "config", "c", "", "Path to configuration file")
+	pflag.IntVarP(&args.MaxMessages, "max-messages", "m", 0, "Maximum number of messages to process per mailbox per run (0 = no limit)")
+	
+	// Add help option
+	pflag.BoolP("help", "h", false, "Show help message")
+	
+	pflag.Parse()
+
+	// Handle help flag
+	if help, _ := pflag.CommandLine.GetBool("help"); help {
+		fmt.Fprintf(os.Stderr, "mail2couch - Email backup utility for CouchDB\n\n")
+		fmt.Fprintf(os.Stderr, "Usage: %s [OPTIONS]\n\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "Options:\n")
+		pflag.PrintDefaults()
+		os.Exit(0)
 	}
+
+	return &args
 }
 
 // FindConfigFile searches for config.json in the following order:
-// 1. Path specified by -config flag
+// 1. Path specified by --config/-c flag
 // 2. ./config.json (current directory)
-// 3. ~/.config/mail2couch/config.json (user config directory)
-// 4. ~/.mail2couch.json (user home directory)
+// 3. ./config/config.json (config subdirectory)
+// 4. ~/.config/mail2couch/config.json (user config directory)
+// 5. ~/.mail2couch.json (user home directory)
 func FindConfigFile(args *CommandLineArgs) (string, error) {
 	if args.ConfigPath != "" {
 		if _, err := os.Stat(args.ConfigPath); err == nil {