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.

docs/turn-loop/README.md

@@ -226,8 +226,6 @@ uid 0 and have the setuid bit set."
   (`nix/agent-modules/agent-service.nix:535`); `packages.hive-agent` is
   wired to the flake's `hyperhive.packages.<system>.hive-agent` output
   (`nix/agent-modules/packages.nix:8-22`) — same binary for every agent.
-- `Restart = on-failure`, `RestartSec = 2` — keeps the harness
-  resilient across transient crashes without thundering retries.
 - `RuntimeDirectory = "hive-config"` → `/run/hive-config/` owned by
   `User=`, autocleared on stop. The harness writes regenerated
   `claude-{mcp-config,settings,system-prompt}` files there