Spec: add generated JSON version and CI check

[bluefuton]

Summary

Adds a JSON version of the spec alongside spec.yml, plus a CI check that keeps the two in sync.

Changes

• Add spec.json, generated from spec.yml with @redocly/cli bundle.
• Add .github/workflows/spec.yml, which on every PR and push to trunk:
  • lints spec.yml
  • regenerates spec.json and fails if it differs from the committed file
• Add redocly.yaml, extending the recommended ruleset but disabling rules that don't fit the Akismet API:
  • security-defined: the API key is a form/query parameter, not an OpenAPI security scheme
  • operation-4xx-response: Akismet returns 200 with plain-text bodies (e.g. invalid) rather than 4xx
  • info-license
• Document the JSON version and regeneration command in the README.

Redocly is pinned to 2.55.0 so local and CI output match.

Testing

• npx @redocly/cli@2.55.0 lint spec.yml passes.
• npx @redocly/cli@2.55.0 bundle spec.yml -o spec.json produces no diff.
• Check the Spec workflow passes on this PR.

Notes

Whichever of this and #5 merges second will need spec.json regenerated, since #5 changes spec.yml.