feat(swarmctl): add agent create, queueing the swarm-controller creation DAG

`swarmctl agent create <name> --hive <hive>` POSTs `/api/agents` to
swarm-controller over the daemon's unix socket and prints the queued
job's node id.

It deliberately does not wait. The endpoint queues a DAG whose last node
*publishes* a deploy message; the hive's `hive-c0re` then converges on
its own clock, out of the controller's sight. So even a fully settled
graph would not mean the agent is up, and there is nothing this CLI
could wait for that would let it claim otherwise. Printing the id is
exactly what the response says and all of what it says.

Transport is a bare hyper HTTP/1.1 client handshaked onto a tokio
`UnixStream` via `hyper_util::rt::TokioIo` — the same crate family
`hivectl/src/watch.rs` and `hive-agent/src/web_ui/proxy.rs` already use,
all of it already workspace-pinned. The request/response shapes are a
local mirror rather than a shared crate: the controller's own types are
private to its binary and this crate does not link it, the same
separation `hivectl` keeps from `hive-c0re`.

Errors are reduced to one actionable line — the controller answers
RFC 9457 problem+json, so an unknown `--hive` reaches the operator as
the roster of hives that would have worked rather than a body dump.
Response `warnings` are printed when non-empty.

The nix module wraps the binary with `SWARM_CONTROLLER_SOCKET`, read
from the same `socketPath` the daemon binds.

Refs #4399

docs/tools/README.md

@@ -21,8 +21,10 @@ debug agent behavior.
   flag-by-flag reference for `swarmctl`, kept in lockstep with the
   binary by CI the same way `hivectl-cli.md` is. `swarmctl` itself
   runs as root on the swarm-controller host, not through `hivectl` —
-  see `swarmctl/README.md` for why. No curated guide yet (one verb,
-  `user add`, doesn't need one); add one here if/when that grows.
+  see `swarmctl/README.md` for why. Two verb families today: `user`
+  (authelia's subject store, edited in place) and `agent create`
+  (queues the swarm-controller's creation job graph). No curated guide
+  yet; add one here if/when that grows.
 
 ## What your agents can do
 

docs/tools/swarmctl-cli.md

@@ -5,6 +5,8 @@ This document contains the help content for the `swarmctl` command-line program.
 **Command Overview:**
 
 * [`swarmctl`↴](#swarmctl)
+* [`swarmctl agent`↴](#swarmctl-agent)
+* [`swarmctl agent create`↴](#swarmctl-agent-create)
 * [`swarmctl user`↴](#swarmctl-user)
 * [`swarmctl user add`↴](#swarmctl-user-add)
 * [`swarmctl user update`↴](#swarmctl-user-update)
@@ -19,6 +21,7 @@ swarm-level operator CLI
 
 ###### **Subcommands:**
 
+* `agent` — Manage agents across the swarm
 * `user` — Manage subjects in the swarm's SSO provider
 * `completions` — Generate a shell completion script for `swarmctl` and print it to stdout
 
@@ -31,6 +34,47 @@ swarm-level operator CLI
 
 
 
+## `swarmctl agent`
+
+Manage agents across the swarm
+
+**Usage:** `swarmctl agent <COMMAND>`
+
+###### **Subcommands:**
+
+* `create` — Queue creation of a new agent on a hive in this swarm
+
+
+
+## `swarmctl agent create`
+
+Queue creation of a new agent on a hive in this swarm.
+
+Asks the swarm-controller to insert its agent-creation job graph — SSO identity, forge user, config repo, and the deploy message that puts the agent on `--hive` — and prints the queued job's node id.
+
+**This returns as soon as the work is queued.** It doesn't wait, and a finished graph would not mean the agent is up either: the last node publishes a deploy, after which the hive converges on its own clock. Watch the swarm UI's job view, or the hive itself, for the rest.
+
+No approval gate guards this: running this binary already means being root on the controller's host.
+
+**Usage:** `swarmctl agent create [OPTIONS] --hive <HIVE> <NAME>`
+
+###### **Arguments:**
+
+* `<NAME>` — Name for the new agent: 1–63 characters of `[a-z0-9-]`.
+
+   Becomes an SSO subject, a forge user and a repository name, so it's validated here before anything is queued.
+
+###### **Options:**
+
+* `--hive <HIVE>` — Hive in this swarm to deploy the agent to.
+
+   Required, and deliberately not defaulted: it's an *address* — the hive a deploy message is sent to — and only the operator knows which one they mean. The controller checks it against the swarm's hive roster and names the known hives if it misses.
+* `--controller-socket <PATH>` — swarm-controller's unix socket.
+
+   Supplied by the nix module that installs this binary, from the same `socketPath` option the daemon binds; falls back to `SWARM_CONTROLLER_SOCKET`.
+
+
+
 ## `swarmctl user`
 
 Manage subjects in the swarm's SSO provider