* Switch CI and Build GhosttyKit to Depot macOS runners
Moves the tests job (unit + UI) and build-ghosttykit job from
self-hosted to depot-macos-latest so CI doesn't block the local
Mac Mini. CI tests now download the pre-built xcframework from
the ghostty releases instead of building with zig. Nightly and
release workflows stay on self-hosted for signing/notarization.
* Add on-demand Depot test workflow, revert CI changes
Adds test-depot.yml (workflow_dispatch) for running tests on Depot
macOS runners during local dev without tying up the Mac Mini.
Reverts ci.yml and build-ghosttykit.yml back to self-hosted.
