Breaking
- Replace the synthetic `other` layer with an unnamed empty-string
  sentinel. Tools expose only `all` plus configured named layers.
- Paths that match no named layer remain indexed and are reachable
  only via `layer=all`. No legacy `other` migration; reindex existing
  indexes after upgrading.
- Config reserves only `all` as a layer name (not `other`).

Docs
- Clarify layer filter semantics for `ask_docs` / `list_docs`.