docs(skills): improve provider package guide (#19573)

## Summary

- rewrite the add-provider-package skill around current repository and
release conventions
- add official OpenAPI discovery guidance with live-response
verification
- cover ProviderV4 factories, workflow serialization, secure URL
handling, repository integration, and npm Trusted Publisher bootstrap
- align AI Functions example and fixture skills with the nested provider
directory convention

## Validation

- `pnpm check`
- `quick_validate.py skills/add-provider-package`
- `quick_validate.py skills/develop-ai-functions-example`
- `quick_validate.py skills/capture-api-response-test-fixture`
- `git diff --check`

No changeset is needed because this only updates internal skill
documentation.