feat: task comments and activity feed

[baskard]

Problem / motivation

Team members need to communicate about tasks without leaving the app. Currently there's no way to add context or updates to a task after creation.

Proposed solution

Add a comments section to each task, visible when the task card is expanded or in a task detail view.

Acceptance criteria

• Each task has a comments section (POST /api/tasks/:id/comments)
• Comments include author name, timestamp, and markdown-formatted body
• Comments are returned in GET /api/tasks/:id/comments (newest first)
• Frontend shows comment count on task card
• Clicking a task opens a detail panel with full comment thread
• Empty state shows "No comments yet — be the first to add context"

Additional context

This is the most-requested feature from user interviews. Will unlock team collaboration use cases.