chore: remove Stainless attribution and infrastructure (#3599)

## Changes

- Remove Stainless attribution from legacy generated-file headers,
README/agent guidance, security reporting, and repository-owned
examples.
- Remove Stainless-only CI/upload infrastructure, obsolete
mirror-specific workflow behavior, and legacy `.stats.yml`.
- Keep Castiron generation metadata, public release workflows, and
mock-server validation intact.

## Out of scope

- Renaming `X-Stainless-*` request headers, retry/idempotency
identifiers, OpenAPI `x-stainless-*` extensions, generated fixture
identifiers, changelog history, or third-party provenance.