remove built-in macOS observer (migrated to solstone-macos native app)
macOS screen/audio capture has moved to the solstone-macos native
companion app which uploads via the remote ingest HTTP API. This
removes the dead Python-based macOS observer subsystem:
- Delete observe/macos/ directory (5 files)
- Delete tests/test_macos_screencapture.py
- Remove observe-macos from CLI registry (sol.py)
- Remove darwin branch from observe/observer.py platform dispatcher
- Remove PyObjC dependencies from pyproject.toml and regenerate uv.lock
- Update docs (OBSERVE.md, CALLOSUM.md, INSTALL.md, README.md, DOCTOR.md)