Adds the full documentation set for the planned desktop pet feature under
docs/features/desktop-pet: README, PRD, user stories, logic design, UI
design, and technical design. The pet is scoped as a lightweight always-on
entry point for quick drafting, drag-and-drop staging, and task status at a
glance; it does not introduce a second Ask/Execute channel, task execution
path, or independent task-state store. Design only, no implementation yet.
