swarm-secret-client: agents may list their own subtree; controller rewrites agent policies

render_agent gains a second stanza: list on
secret/metadata/swarm/agents/<agent>/*, next to the existing read on
secret/data/swarm/agents/<agent>/*. An agent can now learn which
credentials it holds by listing its own subtree. Metadata read, writes
and every other principal's paths stay refused.

An agent's policy was only written when it was minted, so existing agents
would never get the new stanza. swarm-controller now rewrites every
agent's policy at start (read_policy::ensure_agent_policies), with the
same 30s / 24h retry as ensure_hive_access. The roster is the store's
hive-agent-* cert-auth roles, listed with the controller's existing
`list` on auth/cert/certs; the writes use its existing grant on
sys/policies/acl/hive-*. Only the policy is written: mint_and_verify
also reissues the certificate, so the pass does not call it.

Refs #4348

swarm-controller/src/main.rs

@@ -2836,6 +2836,9 @@ async fn main() -> Result<()> {
     // abandoned, since the two of them boot together.
     let hive_names: Vec<String> = hives.iter().map(|h| h.name.clone()).collect();
     read_policy::ensure_hive_access(hive_names).await;
+    // Agents keep the policy they were minted with until this rewrites it, so
+    // what `policy::render_agent` grants today reaches them only from here.
+    read_policy::ensure_agent_policies().await;
 
     let state = AppState {
         hives: Arc::new(hives),