[046] Implement cross-device synchronization

[konard]

Parent epic: #1

Phase: Platform Wrappers
Priority: 46

Scope

Implement opt-in synchronization for safe settings and state across devices without syncing secrets by default.

Implementation Steps

• Build on the settings synchronization design and define sync payload versions.
• Add conflict resolution, encryption, and device identity handling.
• Test sync disablement, merge behavior, and excluded secret fields.

Acceptance Criteria

• Synchronization is off unless the user opts in.
• Secret fields and local-only agent memory are excluded from sync payloads.
• Conflict resolution behavior is deterministic and tested.

Automation

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