docs: drop harness systemd settings; swarm-nats-auth: refuse the hive's shared agent client

Docs (review on #4948): the harness's restart settings are not useful to
a reader, so the `Restart`/`RestartSec` bullet in docs/turn-loop and the
"30 seconds later, no start limit" wording in docs/swarm are removed.
What stays is the behaviour: the harness exits and comes back until the
queue accepts it.

Grant: with the harness's shared-client fallback gone, nothing presents
the hive's shared agent client (`hive-<hive>-agent`) to the queue;
swarm-logs uses that OIDC client over HTTP only. swarm-nats-auth now
refuses it outright instead of granting the configured
`$SWARM.term.{hive}.>` / `$SWARM.agent-state.{hive}.>` publish and its
`_INBOX.client.<id>.>` subscribe, so an agent holding it can no longer
publish as a sibling. The agent arm still runs before the hive arm, so
that id never falls through to its hive's grant.

Removed with it: the `--agent-publish-subject` flag and its two uses in
swarm-nats.nix, `Policy`'s agent subject list, and
swarm-queue-client's `client_inbox_prefix`. The module-eval now asserts
the flag is absent; the policy tests assert the refusal and that the
hive itself is still admitted.

swarm-queue-client/src/agent_icon.rs

@@ -16,9 +16,6 @@
 //!
 //! **Each agent writes its own key** with its own queue credential. The auth
 //! callout grants that credential `$KV.agent-icons.<agent>` and no other key.
-//! The hive's shared agent client gets nothing in this bucket: every agent on
-//! a hive presents that same client, so its grant cannot be narrowed to one
-//! agent's key.
 //!
 //! The writer publishes to [`crate::agent_icon::subject`] directly instead of
 //! opening the bucket, so that one subject is its whole grant. Creating the