swarm-bao: agent certificates issued by a store-generated agent CA

An agent's store identity was signed in swarm-controller's memory by a CA
a controller-host unit generated on disk, and the listener never trusted
that CA. Agent leaves now come from the store itself: a `pki-agents` PKI
mount whose root openbao generates internally, so the agent CA's key
never exists outside the store.

- swarm-bao-agent-pki (new, store host, as the bao granter): enables and
  tunes the mount, generates the root once (guarded on an empty issuer
  list, no replace branch), upserts the `swarm-agent` role (client
  certificates named `hive-agent-*` only, 90 days), caches the CA at
  /var/lib/swarm-bao-tls/agent-ca.pem and composes the listener bundle.
- The listener's tls_client_ca_file is a new listener-client-ca.pem
  (client-ca.pem, then the agent CA). Host cert-auth roles still pin
  client-ca.pem, so an agent leaf satisfies no host role. swarm-bao-certs
  composes the same bundle before openbao starts.
- openbao reads tls_client_ca_file only at start, so when the bundle
  changed after openbao started, swarm-bao-agent-pki restarts
  openbao.service in the container; under `seal = "shamir"` it prints
  the step instead. Once swarm-bao-certs has a cached CA, later boots
  start openbao with it and do not restart.
- The controller policy gains exactly `update` on
  pki-agents/issue/swarm-agent. mint_and_verify now asks that role for
  the leaf (the store generates the key), writes the agent's cert-auth
  role pinning the issuing CA bao returned, and writes the agent's
  policy as render_agent alone: the hive-shared queue credential stanza
  is gone.
- deploy.bao.agentPkiRoleName (must start `swarm-`, asserted with the
  other pki role names); swarm-controller gets
  SWARM_CONTROLLER_AGENT_PKI_MOUNT/_ROLE from the deploy.bao options.

Deleted: swarm-controller-agent-ca and its options (agentCaFile,
agentCaKeyFile), env, LoadCredential entries and assertion;
agent_identity's Authority, rcgen signing and validity window; the
rcgen and time dependencies of swarm-controller (rcgen leaves the
workspace); policy::render_agent_with_queue and its tests. The CN-prefix
assertion policy.rs said was owed is not: agent and host roles pin
different CAs.

Migration is re-creating each agent after deploy; that overwrites the
stale role and policy.

Closes #4756

docs/swarm/secrets.md

@@ -325,6 +325,14 @@ plus one leaf per principal it runs (the table below names the options). Those a
 credentials that can't come out of the store, being what opens it; everything
 else a hive needs does.
 
+Agents are the one principal whose leaf the store issues. Its `pki-agents`
+mount (`deploy.bao.agentPkiMountPath`) holds an agent CA generated inside the
+store, and `swarm-controller`, already logged in under its own host leaf, asks
+that mount's one role for a `hive-agent-<agent>` client certificate at agent
+creation. Host roles never pin that CA and agent roles pin only it, so an
+agent's certificate opens that agent's own `swarm/agents/<agent>/*` and
+nothing else.
+
 ### Per-principal identities
 
 Bao matches a cert-auth role on the certificate's subject, so a certificate is an
@@ -414,7 +422,9 @@ hive domain behind the gateway, down. `network.exposeHostPorts` opens the port
 on the bridge firewall and nowhere else.
 
 Reaching the port grants nothing by itself: openbao answers nothing without a
-client certificate signed by `deploy.bao.clientCaFile`. The passthrough carries
+client certificate signed by `deploy.bao.clientCaFile` or by the store's own
+agent CA. The listener reads both from `listener-client-ca.pem`, which no
+cert-auth role pins. The passthrough carries
 whichever certificate the reader presents, unchanged.
 
 ## The constraint that decides where the root lives