Tasks: optional swimlanes / group-by on kanban (follow-up from #259)

[ReBoticsAI]

Summary

Deferred from epic #259 (Tasks kanban parity with GitHub Projects). Column CRUD, WIP limits, assignee/milestone push, and create/archive lifecycle shipped in that epic. Swimlanes / group-by (Priority, Assignee, etc.) were marked phase later if needed.

Goal

Optional group-by swimlanes on /tasks boards (horizontal lanes), similar to GitHub Projects board group-by.

Acceptance

• Group-by selector (e.g. Priority, Assignee, none)
• Lanes render without breaking column scroll / DnD
• Works on local and GitHub-linked boards
• Docs updated

Out of scope

Full table/roadmap layouts.