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

Cargo.toml

@@ -112,20 +112,6 @@ vaultrs = "0.8"
 # resolves, since its `exec_with_empty` takes *its* `Endpoint` trait.
 rustify = "0.7"
 rustify_derive = "0.5"
-# Signs the per-agent client leaf `swarm-controller::agent_identity` mints.
-# A library rather than an `openssl` shellout, which is what every other CA
-# in this tree is (`glue-bao-tls.nix`, `hive-tls.nix`, `swarm-ca.nix`): those
-# run once at deploy time and write to disk, this one runs per agent
-# creation and must keep the private key it generates in memory long enough
-# to log in with it and no longer. `ring` over `aws_lc_rs` because `ring` is
-# already in the lock; the crypto backend is not otherwise load-bearing.
-#
-# `x509-parser`: `Issuer::from_ca_cert_pem` is gated behind it, and reading
-# the authority back out of its own PEM is how a leaf inherits the subject and
-# key identifier that make it chain. The ungated constructors take a
-# `CertificateParams` the caller would have to restate by hand — two spellings
-# of one authority, agreeing until the day they don't.
-rcgen = { version = "0.14", features = ["x509-parser"] }
 tower-http = { version = "0.7", features = ["fs"] }
 uuid = { version = "1", features = ["v4"] }
 rmcp = { version = "2", default-features = false, features = [