swarm: mint a per-agent queue credential beside the agent's store identity

Every agent on a hive authenticates to the swarm queue with the same
hive-scoped secret today, so at the auth callout one agent is
indistinguishable from its co-hived neighbours and no subject can be
scoped to one of them.

Mint a secret per agent instead, at swarm level, into
secret/swarm/agents/<agent>/queue -- inside the stanza every agent's ACL
document already grants, so no policy changes and no existing agent's
document is rewritten. It is written by the same node that already mints
the agent's certificate, and read back under the agent's own token
before that node reports success.

The secret is not derived from the agent's mTLS identity: the two
credentials answer different questions and coupling their lifetimes
would mean renewing either implied renewing the other. Nothing here
rotates a queue secret -- a re-run keeps the existing value and only
corrects the principal it names, because this function is re-run
deliberately against agents that are already connected. Revoking one
means deleting the path.

Nothing reads the new credential yet; this is the minting half.

swarm-controller/Cargo.toml

@@ -26,6 +26,10 @@ forgejo-api.workspace = true
 # `repo_change_files` calls — forgejo's content API takes base64, never
 # raw bytes.
 base64.workspace = true
+# `agent_identity.rs` mints the per-agent queue secret, the one value in this
+# tree this daemon invents rather than receives. Straight from the kernel's
+# CSPRNG — see the workspace entry for why this and not `rand`.
+getrandom.workspace = true
 futures-util.workspace = true
 # RFC 9457 `application/problem+json` error bodies. Same version + `axum`
 # feature as hive-c0re: the two daemons answer the same operator UIs, so a