Schema: digest vs ciphertext_digest (oneOf/anyOf)

[flyingrobots]

Title
Schema: oneOf/anyOf for digest vs ciphertext_digest (track landed work)

Goal
Align JSON Schema with low‑entropy policy rule: allow pointers that expose only ciphertext_digest.

Acceptance Criteria

• Schema compiles with ajv.
• Examples include a ciphertext‑only pointer.
• CI validation passes.

Status

• Implemented in PR GATOS Privacy: ADR‑0004 + SPEC/TECH‑SPEC + Pointer Schema + gatos‑privacy crate #47; keep this issue for tracking and to add a ciphertext‑only example if needed.