fix(hivectl): ask the daemon whether an agent exists

The agents root is 0700 and owned by the daemon's user, so hivectl's
client-side existence guard hit EACCES on traversal for anyone not root.
It reported that as "this command needs root; re-run with sudo", which
turned three verbs' pre-flight check into a permission error about the
wrong thing: `choom`, `subvol upgrade` and `subvol snapshot create` all
failed at the guard rather than at whatever they actually needed.

The daemon runs as the owning user and already answers this question for
its own provisioning paths, so expose it on the host socket as
`AgentExists` and have hivectl ask. Operators reach that socket through
the `hive-admin` group, so the guard now works without sudo.

`choom` still needs root for `machinectl shell` — we ship no polkit rule
granting those actions — so it now checks the effective uid and says so
directly instead of failing later inside systemd's authorisation.

docs/tools/hivectl.md

@@ -211,7 +211,17 @@ new containers and restarts pick the values up immediately.
 Drop into an interactive Claude session inside an agent container.
 Replaces the current process with `machinectl shell <name>@h-<name>`
 running claude from the agent's state dir. Requires root (same as all
-`machinectl shell` operations).
+`machinectl shell` operations) — hyperhive ships no polkit rule granting
+those actions to the operator group, so `choom` refuses up front with a
+message naming that requirement rather than letting systemd reject the
+exec later.
+
+It also needs the daemon socket, unlike the other exec-into-a-container
+paths: the "is this actually an agent?" pre-flight reads the agents root,
+which is owned by the daemon's user and not group-readable, so the check
+is a `HostRequest` rather than a local `stat`. A rootless `choom` therefore
+tells you it needs root, instead of reporting a permission problem with
+the state dir.
 
 ```bash
 hivectl choom iris                          # fresh blank Claude session in iris's container