Remove dead CI configs (CircleCI + AppVeyor)

[dalberola]

[verified] Two CI configs are dead and misleading; GitHub Actions is the authoritative CI.

• .circleci/config.yml: targets Node 10/12/14 and Python 3.6, calls npx lerna exec (lerna was removed), and runs e2e:apib / e2e:openapi2 scripts that no longer exist.
• appveyor.yml: Windows CI targeting Node 12; not integrated anywhere.

Tasks

• Confirm .github/workflows/* covers test + e2e + docs + publish (it does today).
• Delete .circleci/config.yml.
• Delete appveyor.yml.

[dalberola]

Implemented in #13 (branch chore/remove-dead-ci). Both .circleci/config.yml and appveyor.yml deleted.

Verified GitHub Actions covers build/lint/test (Node 22/24), e2e, docs, and commit-lint. The only two CircleCI checks not yet mirrored — prettify:check and test:smoke — are folded into #10. Will close on merge.