feat(slots): add a retirement workflow and a drift check

Retiring a slot means three separate things in three separate places, and
nothing tied them together.
Alpine 3.22 is what that costs: it was removed from `local.rpkgs_slots` in
`cdn.tf` at some point, so the edge stopped routing it, but it kept a matrix
row in twelve pipelines, six cron jobs on the Crow server and ~50000 objects in
the bucket.
Nightly builds kept producing binaries nobody could reach, and because an
unrouted slot serves the flat index to every client, R 4.4 and R 4.6 users got
`LOAD_FAILED` rather than nothing.

This change will:

- Add `local/check-slots.R`, which fails when any `.crow/*.yaml` still targets
  a slot missing from `local.rpkgs_slots`. Run it with `just check-slots`.
- Add `local/retire-slot.R`, which strips a slot from the pipeline matrices and
  deletes its objects from the bucket. Dry run by default, and it refuses any
  slot still listed in `cdn.tf`, so it cannot be pointed at a live slot.
- Add `.crow/retire-slot.yaml`, which runs the asset half in CI where the B2
  credentials exist.
- Document the ordering in README, including the cron jobs, which live on the
  Crow server and cannot be removed from here.

The check is one-directional on purpose.
A pipeline targeting an unpublished slot is an error; a published slot missing
from *some* pipeline is only reported, because several pipelines cover a subset
deliberately.

Verified against the tree as it stood before #213: the checker names exactly the
twelve files that PR edited by hand, and `retire-slot.R --config-only` reproduces
that commit's structural changes byte for byte across all 18 pipeline files.
The only differences are two prose lines, a usage example and a dropdown
`description:`, which the script leaves alone because rewriting free text
automatically is guesswork.

justfile

@@ -87,3 +87,11 @@ edge-test:
         denoland/deno:latest \
         deno test -A edge/rpkgs-router.test.ts
     fi
+
+# fail if any pipeline still targets a slot the edge does not route
+check-slots:
+    Rscript local/check-slots.R
+
+# strip a retired slot from .crow/*.yaml (dry run; pass --apply to write)
+retire-slot codename *flags:
+    Rscript local/retire-slot.R {{ codename }} --config-only {{ flags }}