docs(providers): register providers removed at their operator's request and guard against reintroduction (#12478)

Adds docs/reference/REMOVED_PROVIDERS.md (policy + register: puter #10210,
the keyless provider removed in #12440), links it from the docs index and
AGENTS.md's provider checklist, and adds a regression test that fails if any
registered id, alias or domain shows up again in the provider catalogs, the
executor map or the registry/executor sources.

Co-authored-by: Markus Hartung <diegosouzapw@users.noreply.github.com>