Document the menu-bar face #19

Last of the menubar-pet stack.

Done when:

• The README shows three faces built, has a phase-plan entry, the workspace layout, launch and Login Items instructions, and the pet's state path with why it is not beside the socket.
• The CHANGELOG has an Unreleased entry.
• The design document matches what was built: 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.
• The Homebrew formula builds and installs the app.