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>

build/custom_xmpp_server.mk

@@ -0,0 +1,72 @@
+# XMPP Development Server Management
+# This file provides targets for managing the development XMPP server
+
+# XMPP server configuration
+XMPP_COMPOSE_FILE := sidecar/docker-compose.yml
+XMPP_ADMIN_URL := http://localhost:9090
+XMPP_SERVER_HOST := localhost
+XMPP_SERVER_PORT := 5222
+
+.PHONY: devserver_start devserver_stop devserver_status devserver_logs devserver_clean devserver_doctor devserver_help
+
+## Start the XMPP development server
+devserver_start:
+	@echo "Starting XMPP development server..."
+	@if [ ! -f "$(XMPP_COMPOSE_FILE)" ]; then \
+		echo "Error: $(XMPP_COMPOSE_FILE) not found"; \
+		exit 1; \
+	fi
+	@cd sidecar && docker compose up -d
+	@echo "✅ XMPP server started successfully"
+	@echo "Admin console: $(XMPP_ADMIN_URL)"
+	@echo "XMPP server: $(XMPP_SERVER_HOST):$(XMPP_SERVER_PORT)"
+	@echo ""
+	@echo "See sidecar/README.md for setup instructions"
+
+## Stop the XMPP development server
+devserver_stop:
+	@echo "Stopping XMPP development server..."
+	@cd sidecar && docker compose down
+	@echo "✅ XMPP server stopped"
+
+## Check status of XMPP development server
+devserver_status:
+	@echo "XMPP Development Server Status:"
+	@cd sidecar && docker compose ps
+
+## View XMPP server logs
+devserver_logs:
+	@echo "XMPP Server Logs (press Ctrl+C to exit):"
+	@cd sidecar && docker compose logs -f openfire
+
+## Clean up XMPP server data and containers
+devserver_clean:
+	@echo "⚠️  This will remove all XMPP server data and containers!"
+	@read -p "Are you sure? (y/N): " confirm && [ "$$confirm" = "y" ] || exit 1
+	@echo "Cleaning up XMPP development server..."
+	@cd sidecar && docker compose down -v --remove-orphans
+	@docker system prune -f --filter label=com.docker.compose.project=sidecar
+	@echo "✅ XMPP server cleaned up"
+
+## Test XMPP client connectivity with doctor tool (insecure for development)
+devserver_doctor:
+	@echo "Testing XMPP client connectivity..."
+	@go run cmd/xmpp-client-doctor/main.go -insecure-skip-verify=true -verbose=true
+
+## Show XMPP development server help
+devserver_help:
+	@echo "XMPP Development Server Targets:"
+	@echo ""
+	@echo "  devserver_start   - Start the XMPP development server"
+	@echo "  devserver_stop    - Stop the XMPP development server"
+	@echo "  devserver_status  - Check server status"
+	@echo "  devserver_logs    - View server logs"
+	@echo "  devserver_clean   - Clean up server data and containers"
+	@echo "  devserver_doctor  - Test XMPP client connectivity"
+	@echo "  devserver_help    - Show this help message"
+	@echo ""
+	@echo "Server Details:"
+	@echo "  Admin Console: $(XMPP_ADMIN_URL)"
+	@echo "  XMPP Server:   $(XMPP_SERVER_HOST):$(XMPP_SERVER_PORT)"
+	@echo ""
+	@echo "See sidecar/README.md for detailed setup instructions"