XY-521: Move progress checkpoints to private-first public projection

[yvette-carlisle]

Summary

• Store full progress checkpoint payloads as private runtime execution events before public writeback.
• Render Linear progress checkpoints as allowlisted public projections and gate repeated private-only updates.
• Update tracker, runtime, and Linear ledger specs for the private-first flow.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test