feat: finish the fleets plugin, add ctx.hosts and host-commands SDK (B4)

packages/plugin-sdk/FINISH-LIST.md

@@ -13,7 +13,13 @@ it's done.
 - **B3 (tailscale):** `plugins/tailscale/src/backend/host-metrics-manager.ts`
   and `routes.ts` still resolve hosts and access through core's
   `PermissionManager`/`resolveHostById`/`DataCrypto` by relative import
-  instead of a `ctx.hosts`-style SDK API, because `ctx.hosts` is not built yet
-  (status **B** in the ctx surface table). Same debt every other host-metrics
-  manager already carries; not new, but tailscale now carries it too. Owner:
-  whichever step builds `ctx.hosts`.
+  instead of `ctx.hosts`, which **B4** built. Same debt every other
+  host-metrics manager already carries; not new, but tailscale now carries it
+  too. Owner: host-metrics's own Phase B step (it and every manager plugin
+  convert together).
+- **B4 (fleets/host-metrics):** `src/backend/hosts/metrics-shared/` (platform
+  detection, package commands, exec-elevated) is still core's own copy, used
+  by host-metrics, which has not converted yet. `@termix/plugin-sdk/host-commands`
+  is the SDK copy fleets now uses; core's copy was left in place rather than
+  turned into a re-export, since host-metrics's own conversion step is where
+  its imports should move wholesale. Owner: host-metrics's Phase B step.