Last of the menubar-pet stack.
README: three faces built rather than two, a phase-plan entry, workspace layout, how to launch the app and add it to Login Items, and the pet's state path with why it sits in data rather than beside the socket.
The design document is corrected to match what shipped: pet.state rather than pet.json, and the two Swift tests that were added rather than the none it predicted. A spec that disagrees with the code is worse than no spec, so it gets corrected rather than left as a record of intent.
Closes #19