docs: remove TASKS.md — all three tasks shipped

- Task 1 (Claude CLI dropdown): shipped in f26fe02.
- Task 2 (OpenAI combobox): shipped in 31c4743b.
- Task 3 (guided source onboarding): shipped in e10320a5.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Empreiteiro