docs: add CEL validation discovery brief

Adds `.agents/context/cel-validation.md`, the umbrella design brief for cross-field validation via Common Expression Language (CEL) on the schema. Frames the design space, surveys prior art, and locks the schema-shape decisions that drove the three Phase-1 implementations already in main: #194 (prefix-overlap lint), #193 (dependentRequired), and #192 (validations key reservation).

The brief covers the engine choice (cel-go), the binding shape (`self.<path>`, matching Kubernetes CRD x-kubernetes-validations and buf/protovalidate idiom), the v0.1.0 lint rule plan (syntax / >=1 field ref / typed-env resolution / native-substitutable detection), the SDK-reach decision (server-only for v0.1.0), the storage and evaluation flow, the security and resource-limit posture, and the phasing for the engine work (Phase 2) plus polish (Phase 3). Open questions section includes the CEL contradiction-detection plan added during the dependentRequired implementation.

Updates `.agents/context/schema-spec.md` to reserve both the `validations:` and `dependentRequired:` keys in the v0.1.0 reserved-keys table and cross-link the brief.

Part of the cross-field validation umbrella (#76). Phase 1 already shipped; this lands the design context so the schema-spec.md link becomes valid on main and Phase 2 contributors have the full design upfront.

Author: zeevdr