Allow deleting tasks from the sidebar without archiving first

Summary

Add a Delete action to the task sidebar right-click menu so users can permanently remove a task without archiving it first.

Problem

I keep my sidebar clean and delete tasks often. Today, deleting a task from the sidebar requires:

1. Right-click → Archive
2. Open Archived tasks
3. Right-click → Delete

That’s slow for a common cleanup flow. Tools like Cursor expose delete directly in the sidebar context menu; PostHog Code should too.

Archiving is useful when you might restore later. For “I’m done, get this out of my list,” archive-then-delete is unnecessary friction.

Desired behavior

• Right-click a task in the sidebar
• See Delete next to Archive / Archive prior tasks
• Confirm in a dialog
• Task is permanently removed (including worktree when present)
• Task disappears from the sidebar immediately

Current behavior

• Sidebar context menu has Pin, Rename, Archive, Archive prior tasks, etc.
• Delete is only available for archived tasks
• Active tasks cannot be deleted directly from the sidebar

Start at the sidebar right-click context menu and compare its existing Archive actions with the Delete action available for archived tasks. Add the active-task Delete entry and confirmation flow, including worktree removal when present. Done means deletion is confirmed, the task is permanently removed, and it disappears from the sidebar immediately.