Introduces a new event-sourced architecture with SQLite persistence:
- CoreEngine handles command dispatch and read model generation
- SqliteEventStore provides durable event storage
- CommandHandlers process domain commands (CreateProject, CreateThread, etc.)
- ReadModels derive current state from event history

Simplifies client-side state management by moving persistence to the server.
Removes redundant client-side persistence schema and session logic tests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>