[P0][security] SharingRuleSchema disconnected from the live engine #1887

Part of the metadata liveness audit umbrella #1878 (P0 security cluster).

Problem

SharingRuleSchema is disconnected from the live engine. The runtime enforces a divergent sys_sharing_rule model:

• runtime: JSON criteria_json, recipient enum user / team / department / role / queue;
• spec: CEL condition, recipients including role_and_subordinates / guest.

The spec's CEL condition is never compiled, and unparsable CEL degrades to "match nothing" — so spec-authored sharing rules silently grant no access. The two models have diverged into a write-only spec surface.

Decision required (enforce or remove)

• Reconcile/enforce: compile the spec's CEL condition into the live engine and map the spec recipient set (including role_and_subordinates, which depends on the Role parent walk, and guest) onto sys_sharing_rule.
• Remove: deprecate SharingRuleSchema in favor of authoring sys_sharing_rule directly, and document the supported (criteria_json, 5-recipient) model.

Evidence

• docs/audits/2026-06-security-identity-property-liveness.md
• runtime sys_sharing_rule model vs spec SharingRuleSchema

Note: depends on the Role parent hierarchy issue for the role_and_subordinates recipient.