ALL-2003: Expose condition target fields + validate requirement conditions

Backend work for stage requirement conditional visibility. Add availableConditionTargetFields query, ensure updateRequirement persists/clears condition (pass condition:null to remove), treat requirements with empty/missing condition target as inapplicable (not blocking) in computeStateStatus, and emit validation warnings for unknown factKeys. Files: domains/program/src/services/lifecycle-definition/entities.ts, domains/program/src/services/stage-requirements/updateRequirement.ts, domains/program/src/services/lifecycle-instance/predicates.ts, domains/program/src/services/lifecycle-instance/computeStateStatus.ts, domains/program/src/subgraph/schema.graphql. Out of scope: compound conditions, circular dep detection, frontend UI.