## Motivation
`weekly-rebuild-reindex` exists to run after `weekly-rebuild-missing`: it declares `depends_on: weekly-rebuild-missing` and is gated on that workflow's `weekly_rebuild_missing` variable. Triggering it manually therefore also starts hours of package rebuilds.
That is the wrong tool when only the index needs regenerating. After rpkgs/bincraft#113 (v5.1.5), which changes how `union_index_records()` decides what a per-minor index steers to, every object in the bucket is already correct and only `PACKAGES*` is stale. Rebuilding to fix an index is pure waste, and the natural cron would take a full cycle to reach every slot.
## Change
Adds `.crow/reindex.yaml`: the index half on its own, manual only, no dependency on a rebuild.
It reuses the same matrix and the same steps as `weekly-rebuild-reindex` — install the latest bincraft release, republish the generic index, loop the installed R versions republishing each per-minor index, purge the edge. No package is built.
Gated on a new `reindex` variable so it cannot be started by the rebuild gate, defaulting to `none` so a manual pipeline creation (which instantiates every file in `.crow/`) matches no matrix row.
```sh
crow pipeline create devxy/build-cran-binaries --var reindex=all
crow pipeline create devxy/build-cran-binaries --var reindex=ubuntu-2404-amd64
```
## Verification
- `crow lint .crow/` passes.
- Gate is manual-only and evaluates `reindex`, with no `depends_on` and no `runs_on` carried over from the rebuild coupling.
Reviewed-on: #177
