[Task]: Implement chat discussions and threads for tasks with @ mentions

[luizhcastro]

Objective

Implement a comprehensive discussion and threading system for tasks that allows team members to communicate effectively through nested comments, @mentions for user notifications, and #references for linking to related issues, PRs, and other tasks.

Scope

In scope:

Backend Implementation:

• Create comments entity with fields: task_id, user_id, content, parent_comment_id (for threading)
• POST /api/tasks/:id/comments endpoint for creating comments
• POST /api/comments/:id/replies endpoint for creating threaded replies
• GET /api/tasks/:id/comments returns nested threads structure
• DELETE /api/comments/:id endpoint for comment deletion
• PUT /api/comments/:id endpoint for editing comments
• Store mentioned users in mentions table for tracking

Mention System:

• @mention parsing logic to detect user mentions in comments
• User search/autocomplete API for @mentions
• Notification system for mentioned users

Reference System:

• #reference parsing logic to detect and link issues/PRs/tasks
• Support for #123 format to reference issues and PRs within repository
• Automatic linking and preview of referenced items
• Store references in database for relationship tracking

Frontend Implementation:

• Markdown rendering support for comments with @mentions and #references
• Rich text editor with autocomplete for @mentions and #references
• Threaded reply UI with proper nesting visualization
• Comment actions: edit, delete, reply

Testing & Documentation:

• Unit tests for comment CRUD and threading logic
• Unit tests for @mention and #reference parsing
• API documentation updated with new endpoints

Out of scope:

• Real-time notifications for mentions (separate issue)
• UI component to display timeline (separate issue)
• Emoji reactions to comments
• Comment attachments/file uploads
• Comment search functionality

Related Issues

• Blocked by: [Task]: Implement milestones #54, [Task]: Implement relationships/dependencies between tasks #55
• Related to: [Task]: Implement history and logs on task #56, [Task]: Refactor task page details UI #58