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>

sidecar/README.md

@@ -1,17 +1,142 @@
 # Development XMPP Server
 
-This folder contains a `docker-compose.yml` file for development purposes. It sets up a local XMPP server to use while developing the Mattermost XMPP bridge plugin.
+This folder contains a `docker-compose.yml` file for development purposes. It sets up a local XMPP server (Openfire) to use while developing the Mattermost XMPP bridge plugin.
 
-## Usage
+## Quick Start
 
-To start the development XMPP server:
+From the project root directory, use the Makefile targets:
 
 ```bash
-docker-compose up -d
+# Start the XMPP server
+make devserver_start
+
+# Check server status
+make devserver_status
+
+# Test connectivity with the doctor tool
+make devserver_doctor
+
+# Stop the server
+make devserver_stop
 ```
 
-To stop the server:
+## Manual Docker Usage
+
+Alternatively, you can manage the server directly:
 
 ```bash
-docker-compose down
-```
+# Start the server
+cd sidecar
+docker compose up -d
+
+# Stop the server
+docker compose down
+
+# View logs
+docker compose logs -f openfire
+```
+
+## Initial Setup
+
+After starting the server for the first time, you need to complete the Openfire setup:
+
+### 1. Access Admin Console
+
+Open your web browser and go to: http://localhost:9090
+
+### 2. Complete Setup Wizard
+
+1. **Language Selection**: Choose your preferred language
+2. **Server Settings**: 
+   - Server Domain: `localhost` (default is fine)
+   - Keep other defaults
+3. **Database Settings**: 
+   - Choose "Embedded Database" for development
+   - This creates a local database that persists in Docker volumes
+4. **Profile Settings**: 
+   - Choose "Default" (no LDAP needed for development)
+5. **Administrator Account**:
+   - Username: `admin`
+   - Password: `admin` (for development consistency)
+   - Email: `admin@localhost`
+
+### 3. Create Test User
+
+After completing the setup wizard:
+
+1. Log in to the admin console with `admin` / `admin`
+2. Go to **Users/Groups** → **Create New User**
+3. Fill in the user details:
+   - **Username**: `testuser`
+   - **Password**: `testpass`
+   - **Confirm Password**: `testpass`
+   - **Name**: `Test User`
+   - **Email**: `testuser@localhost`
+4. Click **Create User**
+
+### 4. Test Connectivity
+
+Run the doctor tool to verify everything is working:
+
+```bash
+make devserver_doctor
+```
+
+You should see successful connection, ping, and disconnect messages.
+
+## Server Details
+
+- **Admin Console**: http://localhost:9090
+- **XMPP Server**: localhost:5222 (client connections)
+- **XMPP SSL Server**: localhost:5223 (SSL client connections)
+- **XMPP Server-to-Server**: localhost:5269
+- **File Transfer Proxy**: localhost:7777
+
+## Test Credentials
+
+After setup, use these credentials for testing:
+
+- **Admin User**: `admin` / `admin`
+- **Test User**: `testuser@localhost` / `testpass`
+
+## Data Persistence
+
+The server data is stored in Docker volumes:
+- `sidecar_openfire_data`: Openfire configuration and database
+- `sidecar_postgres_data`: PostgreSQL database (if you choose PostgreSQL instead of embedded DB)
+
+## Troubleshooting
+
+### Server Won't Start
+```bash
+# Check if ports are already in use
+lsof -i :9090
+lsof -i :5222
+
+# View server logs
+make devserver_logs
+```
+
+### Reset Everything
+```bash
+# This removes all data and containers
+make devserver_clean
+```
+
+### Test Different Configurations
+```bash
+# Test with custom server settings
+go run cmd/xmpp-client-doctor/main.go \
+  -server="localhost:5222" \
+  -username="testuser@localhost" \
+  -password="testpass" \
+  -insecure-skip-verify=true \
+  -verbose=true
+```
+
+## Development Notes
+
+- The server uses self-signed certificates, so the doctor tool defaults to `-insecure-skip-verify=true`
+- All data persists between container restarts unless you run `make devserver_clean`
+- The PostgreSQL and Adminer services are included but optional (you can use embedded database)
+- The server takes ~30 seconds to fully start up after `docker compose up`