docs: drop stale references to the cranlike fork

- explain GITHUB_PAT by bincraft's DESCRIPTION reads from the CRAN mirror
- describe how upload_package_index() builds the index in the README

.crow/backfill-flat-safety.yaml

@@ -140,12 +140,11 @@ steps:
         from_secret: PGPASS
       REPO_RO_TOKEN:
         from_secret: REPO_RO_TOKEN
-      # uvr resolves bincraft's dependencies through the GitHub API, including
-      # the pat-s/cranlike@s3 fork. Unauthenticated it is rate limited, and the
-      # failure is not loud: uvr warns, falls back to the registry copy of
-      # cranlike, and the index write then dies on `unused argument (built =
-      # built)` because upstream has no such parameter. Intermittent by nature,
-      # since it depends on the rate-limit window.
+      # bincraft reads the DESCRIPTION of every new or replaced tarball from
+      # the CRAN mirror on raw.githubusercontent.com, and authenticates with
+      # this token when it is set. A slot whose published index cannot be read
+      # is re-read in full, ~25k requests, which the anonymous rate limit would
+      # cut short.
       GITHUB_PAT:
         from_secret: GITHUB_PAT
       GIT_USER: pat-s