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>

server/plugin.go

@@ -5,8 +5,11 @@ import (
 	"sync"
 	"time"
 
+	mattermostbridge "github.com/mattermost/mattermost-plugin-bridge-xmpp/server/bridge/mattermost"
+	xmppbridge "github.com/mattermost/mattermost-plugin-bridge-xmpp/server/bridge/xmpp"
 	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/command"
 	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/store/kvstore"
+	"github.com/mattermost/mattermost-plugin-bridge-xmpp/server/xmpp"
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/plugin"
 	"github.com/mattermost/mattermost/server/public/pluginapi"
@@ -27,9 +30,15 @@ type Plugin struct {
 	// commandClient is the client used to register and execute slash commands.
 	commandClient command.Command
 
+	// xmppClient is the client used to communicate with XMPP servers.
+	xmppClient *xmpp.Client
+
 	// logger is the main plugin logger
 	logger Logger
 
+	// remoteID is the identifier returned by RegisterPluginForSharedChannels
+	remoteID string
+
 	backgroundJob *cluster.Job
 
 	// configurationLock synchronizes access to the configuration.
@@ -38,6 +47,10 @@ type Plugin struct {
 	// configuration is the active plugin configuration. Consult getConfiguration and
 	// setConfiguration for usage.
 	configuration *configuration
+
+	// Bridge components for dependency injection architecture
+	mattermostToXMPPBridge *mattermostbridge.MattermostToXMPPBridge
+	xmppToMattermostBridge *xmppbridge.XMPPToMattermostBridge
 }
 
 // OnActivate is invoked when the plugin is activated. If an error is returned, the plugin will be deactivated.
@@ -49,6 +62,11 @@ func (p *Plugin) OnActivate() error {
 
 	p.kvstore = kvstore.NewKVStore(p.client)
 
+	p.initXMPPClient()
+
+	// Initialize bridge components
+	p.initBridges()
+
 	p.commandClient = command.NewCommandHandler(p.client)
 
 	job, err := cluster.Schedule(
@@ -85,4 +103,23 @@ func (p *Plugin) ExecuteCommand(c *plugin.Context, args *model.CommandArgs) (*mo
 	return response, nil
 }
 
+func (p *Plugin) initXMPPClient() {
+	config := p.getConfiguration()
+	p.xmppClient = xmpp.NewClient(
+		config.XMPPServerURL,
+		config.XMPPUsername,
+		config.XMPPPassword,
+		config.GetXMPPResource(),
+		p.remoteID,
+	)
+}
+
+func (p *Plugin) initBridges() {
+	// Create bridge instances (Phase 4 will add proper dependencies)
+	p.mattermostToXMPPBridge = mattermostbridge.NewMattermostToXMPPBridge()
+	p.xmppToMattermostBridge = xmppbridge.NewXMPPToMattermostBridge()
+	
+	p.logger.LogInfo("Bridge instances created successfully")
+}
+
 // See https://developers.mattermost.com/extend/plugins/server/reference/