Add embedded replica and sync mode

[onsails]

Context

Future work from docs/superpowers/specs/2026-05-24-local-libsql-migration-design.md.

The local libsql migration and Turso cloud configuration model are prerequisites. This issue should build on the right-db driver boundary instead of introducing driver-specific APIs in domain crates.

Scope

• Add an embedded replica mode using local reads and remote-backed sync semantics.
• Define explicit sync triggers and startup behavior.
• Preserve local-only mode as the default for existing agents.
• Document consistency expectations, including read-your-writes behavior if enabled.

Acceptance Criteria

• Existing agents keep working without cloud configuration.
• Replica mode is configured through the approved storage config model.
• Sync errors surface through structured errors/logs rather than being swallowed.
• Tests cover local-only mode and replica-mode connection construction.

Out Of Scope

• Backup/restore UX.
• Doctor sync health thresholds.
• Migration tooling from local-only data to cloud primary.