Propose a local-first device database

Adds docs/decisions/2026-11-01-local-first-device-database.md for review. It proposes one SQLite event store per Mac owned by the executor, sessions bound to a stable folder and provider thread id so an executor restart resumes the conversation, local-only storage as the default, and the cloud reduced to a session index plus Move for several Macs. The work is phased so each step ships on its own, and phase 1 fixes the restart bug. See the Phases and Open questions sections.