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/bridge/mattermost/bridge.go

@@ -0,0 +1,11 @@
+package mattermost
+
+// MattermostToXMPPBridge handles syncing messages from Mattermost to XMPP
+type MattermostToXMPPBridge struct {
+	// TODO: Implement in Phase 4
+}
+
+// NewMattermostToXMPPBridge creates a new Mattermost to XMPP bridge
+func NewMattermostToXMPPBridge() *MattermostToXMPPBridge {
+	return &MattermostToXMPPBridge{}
+}