feat: JSON Schema response validation core (#41)

[rwrife]

Closes #41 (partially — see follow-ups).

What changed

Pure, VS Code-free core for validating response bodies against a JSON Schema referenced by a new # @schema directive.

• src/core/schemaRef.ts — RFC 6901 JSON Pointer parser/resolver + in-document $ref inliner (circular-ref safe).
• src/core/schemaValidator.ts — parses @schema into a SchemaRef (inline JSON, openapi:<path>#/<pointer>, or file:<path>[#/<pointer>]), resolves it against caller-supplied loaded docs, and validates a body with ajv + ajv-formats. Non-JSON responses are reported as skipped, not failed, matching the acceptance criteria.
• Parser already generalizes # @<key> <value> into request.directives, so # @schema … is captured with zero parser changes; new tests pin that contract.
• README — new "Schema validation" section with inline + OpenAPI-ref examples.
• Deps — ajv + ajv-formats (only these; no ajv-cli, no json-schema-ref-parser, per the issue).

Tests

+32 unit tests (test/schemaRef.test.ts, test/schemaValidator.test.ts). All 398 tests pass. Lint + typecheck clean. npm run build clean.

Follow-ups (still open on #41, intentionally out of scope here to keep this PR small and reviewable)

• Response-panel Schema tab (pass badge / violations list / copy-as-issue-body).
• CLI (reqit run) exit-code wiring so schema failures gate CI.
• Extension-side path guarding for openapi:/file: doc loads (src/core/pathGuard.ts already exists — trivial once the panel tab lands).

Design notes

• Kept all IO out of src/core/ (loaded OpenAPI/JSON Schema docs are passed in via SchemaResolveContext). The extension layer picks the file-reading strategy, and the CLI can do the same without any code duplication.
• Compiled ajv validators are cached in a WeakMap keyed on the schema object, so re-running the same .http file doesn't recompile.