swarm: courier an agent's store identity into its container, and log in with it

`swarm-controller` mints an agent's mTLS leaf at creation and publishes it
at `swarm/agents/<agent>/bao-mtls`. Nothing read it back. This adds the
hop that carries it the rest of the way, and the in-container consumer
that proves the hop works.

Host side, `lifecycle::agent_identity` reads the row under *this hive's*
own certificate — the hive is a principal the store already knows — and
stages the leaf and its key `0600` under a new `agent-identity/<name>`
state dir, deliberately outside every bind-mounted tree. Both files go in
as systemd credentials rather than binds, the same answer and the same
mode reason as the queue secret beside it: the staged key is unreadable
to the unprivileged agent user, and the container manager reads a
`--load-credential` source as root before re-exposing it under the
consuming unit's own `User=`. The agent is never asked to authenticate in
order to obtain the thing it authenticates with.

Container side, `hive-agent-bao-identity.service` logs in with that
certificate and reads the agent's own path back, failing the unit when
either step does not succeed. It fails loudly where the hive-side readers
degrade quietly, because a refused certificate means an agent that
believes it reaches the store and never does — a cause only the login
itself can name.

The address is the whole switch, no separate `enable`, matching how
`queue.nix` and `logs.nix` already gate themselves. A hive with a store
forwards `HIVE_AGENT_BAO_ADDR` and every agent on it gets the check; a
hive without one forwards nothing and no agent does. That is what keeps
the delivery from landing in a container with nothing to read it.

The hive can now reach an agent's identity, so hive privilege covers
agent privilege. Accepted, not mitigated: the alternative is an agent
fetching its own credential with a credential it does not yet have.

Refs #4137

nix/agent-modules/default.nix

@@ -40,6 +40,7 @@ in
 {
   imports = [
     ./agent-service.nix
+    ./bao.nix
     ./bash-env.nix
     ./claude-settings.nix
     ./dashboard-links.nix