Add robust external response.schema_ref support (JSON + Zod modules) with metadata, validation, and tests

[PredictabilityAtScale]

Motivation

• Let prompts reference a single source-of-truth schema file so teams can author schemas outside YAML and reuse them in prompts and runtime code.
• Support both simple JSON Schema files and code-first Zod schemas while keeping a safe, discoverable, and debuggable load/compile/validate UX.
• Harden error handling and DX (actionable error codes, inspectable provenance, and provider-specific warnings) to avoid confusing filesystem or import failures.

Description

• Added resolver src/parser/response-schema-ref.ts and wired it into the prompt loader so response.schema_ref is resolved relative to the prompt and normalized into response.schema with response.schema_ref removed.
• Supported file types: .json (parsed as JSON Schema) and .js/.mjs/.cjs (imported modules exporting a Zod schema converted to JSON Schema via zod-to-json-schema), and enforced a whitelist of allowed extensions; .ts/.tsx are intentionally not supported by default.
• Emit and populate response.schema_source metadata (mode, ref, resolved_path, hash) so inspect and compiled artifacts can show schema provenance.
• Enforced mutual exclusion of response.schema and response.schema_ref in runtime schema definitions and validation (ResponseSchemaWithValidation).
• Improved error handling with clear, feature-specific error codes (POK050 for JSON ref errors, POK051 for module/ref import/export/extension issues) and added provider-sensitive schema keyword warnings (POK052/POK053/POK054).
• Updated docs (docs/schema-dx.md, docs/schema.md, README.md, docs/index.md, docs/overrides.md) to document schema_ref usage, constraints, and the executable-module trust boundary; fixed a promptops → promptopskit wording drift.
• Added zod-to-json-schema dependency and comprehensive tests covering happy paths and negative cases (invalid JSON, missing module file, module without Zod export, unsupported extensions), and fixed related test organization/regressions.

Testing

• Ran unit test suite with npm test (Vitest); all tests passed: 13 test files, 221 tests (all green).
• Ran static typecheck/lint via npm run lint (tsc --noEmit); the command completed successfully.
• Added/updated integration tests that assert: inspect exposes response.schema_source, validate/compile resolve prompt-relative JSON refs, compile inlines resolved response.schema and removes schema_ref, and failure modes for invalid JSON, missing Zod modules, modules without a Zod export, and unsupported extensions cause the expected CLI validation errors.

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Codex Task