[feature]: Mattermost Integration

Is there an existing issue for this? • I have searched the existing issues Summary Feature Request: Mattermost Integration for Plane Overview Add native Mattermost integration to Plane to enable real-time notifications and issue management through Mattermost. Key Features Priority 1: Essential Notifications • Notify when user is mentioned in an issue/project • Notify when someone comments on an issue where user is assignee • Configurable per-channel notification settings • Preview of issue details in notifications Priority 2: Issue Management • Create new issues directly from Mattermost via /plane create • Link Mattermost threads to Plane issues for contextual discussions • Basic issue actions (assign, comment) through Mattermost commands Technical Scope • Secure authentication between Plane and Mattermost • Simple setup process for workspace admins • Rate limiting for notifications Why sho…