feat: implement XMPP client and development server infrastructure

## XMPP Client Implementation
- Create XMPP client with mellium.im/xmpp library
- Add SASL Plain authentication with TLS support
- Implement basic connection, ping, and disconnect functionality
- Add TLS certificate verification skip option for development

## Development Server Management
- Add custom makefile targets for XMPP server management
- Implement devserver_start, devserver_stop, devserver_status commands
- Add devserver_logs, devserver_clean, devserver_doctor commands
- Create comprehensive sidecar/README.md with setup instructions

## XMPP Client Doctor Tool
- Create cmd/xmpp-client-doctor diagnostic tool
- Add CLI flags for server configuration with sensible defaults
- Implement verbose logging and connection testing
- Include insecure TLS option for development environments

## Bridge Architecture Foundation
- Create placeholder bridge structs in proper package hierarchy
- Add server/bridge/mattermost and server/bridge/xmpp packages
- Update plugin initialization to create bridge instances
- Maintain clean separation between Mattermost and XMPP concerns

## Dependencies and Configuration
- Add mellium.im/xmpp dependencies to go.mod
- Fix plugin.json password field type validation
- Update README.md with XMPP bridge description and doctor usage
- Add .claude.md to .gitignore for local development notes

All tests passing. Ready for Phase 4 (Bridge Logic) implementation.

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

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

cmd/xmpp-client-doctor/main.go

@@ -0,0 +1,165 @@
+package main
+
+import (
+	"crypto/tls"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"time"
+
+	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/xmpp"
+)
+
+const (
+	// Default values for development server (sidecar)
+	defaultServer   = "localhost:5222"
+	defaultUsername = "testuser@localhost"
+	defaultPassword = "testpass"
+	defaultResource = "doctor"
+)
+
+type Config struct {
+	Server             string
+	Username           string
+	Password           string
+	Resource           string
+	Verbose            bool
+	InsecureSkipVerify bool
+}
+
+func main() {
+	config := &Config{}
+
+	// Define command line flags
+	flag.StringVar(&config.Server, "server", defaultServer, "XMPP server address (host:port)")
+	flag.StringVar(&config.Username, "username", defaultUsername, "XMPP username/JID")
+	flag.StringVar(&config.Password, "password", defaultPassword, "XMPP password")
+	flag.StringVar(&config.Resource, "resource", defaultResource, "XMPP resource")
+	flag.BoolVar(&config.Verbose, "verbose", true, "Enable verbose logging")
+	flag.BoolVar(&config.InsecureSkipVerify, "insecure-skip-verify", true, "Skip TLS certificate verification (for development)")
+
+	flag.Usage = func() {
+		fmt.Fprintf(os.Stderr, "xmpp-client-doctor - Test XMPP client connectivity\n\n")
+		fmt.Fprintf(os.Stderr, "This tool tests the XMPP client implementation by connecting to an XMPP server,\n")
+		fmt.Fprintf(os.Stderr, "performing a connection test, and then disconnecting gracefully.\n\n")
+		fmt.Fprintf(os.Stderr, "Usage:\n")
+		fmt.Fprintf(os.Stderr, "  %s [flags]\n\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "Flags:\n")
+		flag.PrintDefaults()
+		fmt.Fprintf(os.Stderr, "\nDefault values are configured for the development server in ./sidecar/\n")
+		fmt.Fprintf(os.Stderr, "Make sure to start the development server with: cd sidecar && docker-compose up -d\n")
+	}
+
+	flag.Parse()
+
+	if config.Verbose {
+		log.SetFlags(log.LstdFlags | log.Lmicroseconds)
+		log.Printf("Starting XMPP client doctor...")
+		log.Printf("Configuration:")
+		log.Printf("  Server: %s", config.Server)
+		log.Printf("  Username: %s", config.Username)
+		log.Printf("  Resource: %s", config.Resource)
+		log.Printf("  Password: %s", maskPassword(config.Password))
+	}
+
+	// Test the XMPP client
+	if err := testXMPPClient(config); err != nil {
+		log.Fatalf("❌ XMPP client test failed: %v", err)
+	}
+
+	if config.Verbose {
+		log.Printf("✅ XMPP client test completed successfully!")
+	} else {
+		fmt.Println("✅ XMPP client connectivity test passed!")
+	}
+}
+
+func testXMPPClient(config *Config) error {
+	if config.Verbose {
+		log.Printf("Creating XMPP client...")
+	}
+
+	// Create XMPP client with optional TLS configuration
+	var client *xmpp.Client
+	if config.InsecureSkipVerify {
+		if config.Verbose {
+			log.Printf("Using insecure TLS configuration (skipping certificate verification)")
+		}
+		tlsConfig := &tls.Config{
+			InsecureSkipVerify: true,
+		}
+		client = xmpp.NewClientWithTLS(
+			config.Server,
+			config.Username,
+			config.Password,
+			config.Resource,
+			"doctor-remote-id",
+			tlsConfig,
+		)
+	} else {
+		client = xmpp.NewClient(
+			config.Server,
+			config.Username,
+			config.Password,
+			config.Resource,
+			"doctor-remote-id",
+		)
+	}
+
+	if config.Verbose {
+		log.Printf("Attempting to connect to XMPP server...")
+	}
+
+	// Test connection
+	start := time.Now()
+	err := client.Connect()
+	if err != nil {
+		return fmt.Errorf("failed to connect to XMPP server: %w", err)
+	}
+	connectDuration := time.Since(start)
+
+	if config.Verbose {
+		log.Printf("✅ Connected to XMPP server in %v", connectDuration)
+		log.Printf("Testing connection health...")
+	}
+
+	// Test connection health
+	start = time.Now()
+	err = client.TestConnection()
+	if err != nil {
+		return fmt.Errorf("connection health test failed: %w", err)
+	}
+	pingDuration := time.Since(start)
+
+	if config.Verbose {
+		log.Printf("✅ Connection health test passed in %v", pingDuration)
+		log.Printf("Disconnecting from XMPP server...")
+	}
+
+	// Disconnect
+	start = time.Now()
+	err = client.Disconnect()
+	if err != nil {
+		return fmt.Errorf("failed to disconnect from XMPP server: %w", err)
+	}
+	disconnectDuration := time.Since(start)
+
+	if config.Verbose {
+		log.Printf("✅ Disconnected from XMPP server in %v", disconnectDuration)
+		log.Printf("Connection summary:")
+		log.Printf("  Connect time: %v", connectDuration)
+		log.Printf("  Ping time: %v", pingDuration)
+		log.Printf("  Disconnect time: %v", disconnectDuration)
+		log.Printf("  Total time: %v", connectDuration+pingDuration+disconnectDuration)
+	}
+
+	return nil
+}
+
+func maskPassword(password string) string {
+	if len(password) <= 2 {
+		return "****"
+	}
+	return password[:2] + "****"
+}