[feature] Response schema validation (JSON Schema / OpenAPI ref)

[rwrife]

Pitch

We already import OpenAPI 3.x specs (#25) and support # @test assertions (#6). Close the loop: let devs assert responses against a schema instead of hand-writing per-field assertions. Two entry points, one core:

1. Inline JSON Schema — # @schema { "type": "object", "required": ["id"] }
2. Reference a schema from an imported OpenAPI file — # @schema openapi:./api.yaml#/paths/~1users~1{id}/get/responses/200

Assertion runs after the response comes back; failures show up in the response panel with the exact JSON Pointer path and a plain-English reason.

Why this matters

• API contract drift is the [M1] Scaffold VS Code extension + send a single request #1 silent bug source. Reqit already has the response in hand; validating it is table-stakes for anyone wiring .http files into CI ([M6] Run-file mode + test assertions + CLI #6 CLI mode).
• Zero new heavy deps: use ajv (already common in the TS ecosystem, MIT-licensed, tiny footprint, we already validate everything with zod — ajv earns its keep because it is the JSON Schema validator).
• Reuses the existing OpenAPI import path — no second parser.

User-visible behavior

• Add # @schema <inline-json | openapi:<path>#/<pointer> | file:<path>> directive above a request.
• On successful response, validate response.body against the resolved schema.
• Failures rendered in a new Schema tab in the response panel:
  • Bulleted list of violations (JSON Pointer + rule + expected/actual)
  • Copy-as-issue-body button
• Passes: green checkmark badge on the response panel header.
• CLI (reqit run) exits non-zero on schema-validation failure — same as @test failures — so this is drop-in for CI gates.

Acceptance criteria

• Pure src/core/schemaValidator.ts: resolves a schema ref (inline / file / openapi-pointer), validates a JSON body with ajv, returns a normalized { ok, violations[] } result. Unit-tested with vitest (inline pass/fail, ref resolution, invalid-pointer errors, non-JSON response guard).
• Pure src/core/schemaRef.ts: JSON Pointer + OpenAPI $ref resolver (walks components.schemas and paths.*.responses.*.content['application/json'].schema). Unit-tested.
• Parser (src/core/parser.ts) recognizes # @schema directive; validated by zod; ignored on non-JSON responses with a warning.
• Response panel gets a Schema tab (only shown when a @schema was set) with pass/fail + violations list.
• CLI: schema-validation failure participates in the run summary + non-zero exit.
• README: short "Schema validation" section with an inline example and an OpenAPI-ref example.
• Dependency addition: ajv + ajv-formats (only these; no ajv-cli, no json-schema-ref-parser).

[rwrife]

Kicked off with #44 — pure core (schemaValidator + schemaRef), @schema directive recognition, ajv + ajv-formats deps, README section, +32 unit tests. Response-panel Schema tab and CLI exit-code wiring intentionally left for follow-up PRs to keep #44 small and reviewable.

[rwrife]

Merged the schema-validation core in #44. Response-panel Schema tab, CLI exit-code wiring, and path-guard hookup remain as follow-ups tracked on this issue.