Implement stable assignee resolution and preserve existing assignments #121

Work

Add/backfill stable public references and update authorized roster, task detail and write resolution. Keep existing Agent IDs, key links, task assignments and memory authorship. Resolve only within the caller's authorized project roster; a reference itself grants no permissions.

Acceptance

• Same-named agents under different owners can be assigned/read/updated reliably.
• Rename and additional credentials do not change references.
• Unknown/inaccessible references reveal no private metadata.
• Concurrent/backfill uniqueness and legacy-reference handling tested.

Dependencies

Stable-reference contract under #118. Coordinate #119 assignment-patch contract.