agent capabilities live in a swarm KV record, set from swarmctl

An agent's capabilities move from the hive's meta repo
(`meta/capabilities.json`, injected as `HIVE_CAPABILITIES`) into the
`capabilities` field of its key in a new `agent-permissions` KV bucket,
which swarm-controller alone writes.

- swarm-queue-client: `agent_permissions` (bucket, key, `AgentPermissions`,
  direct-get read, the import subject and its wire types).
- swarm-controller: creates the bucket at start; `GET|PUT|DELETE
  /api/agents/{name}/permissions` (deny_unknown_fields and unknown
  capability ⇒ 422, duplicate ⇒ 400, agent must be declared by a hive);
  answers each hive's one-shot import, writing a key only where there is
  none and only for agents that hive's wanted state declares.
- swarmctl: `agent permissions get|set|clear`.
- swarm-nats-auth: a hive reads every key of the bucket and sends its
  import on its own subject; the controller subscribes to the imports and
  may answer them (`responses`).
- hive-c0re: reads each of its agents' keys every 30s into
  /var/lib/hyperhive/swarm-agent-permissions.json and restarts an agent,
  gracefully, when its capabilities change. Unset is the compiled-in
  default (ruth: manage_root_agent; everyone else: none). A failed read
  keeps the stored value. On first boot it seeds that file from
  meta/capabilities.json, sends the grants to the controller, and deletes
  the file once the controller answers.

Deleted: the capabilities.json reader/writer, `HIVE_CAPABILITIES` in the
rendered meta flake, ruth's capability seed, the capabilities half of the
permissions rebuild job (`PermPayload::Capabilities`/`Combined`), the
dashboard's capabilities table and `capabilities_changed` event, and the
now-unused `Capability::description`.

Part of #5170.

docs/tools/swarmctl-cli.md

@@ -12,6 +12,10 @@ This document contains the help content for the `swarmctl` command-line program.
 * [`swarmctl agent inference get`↴](#swarmctl-agent-inference-get)
 * [`swarmctl agent inference set`↴](#swarmctl-agent-inference-set)
 * [`swarmctl agent inference clear`↴](#swarmctl-agent-inference-clear)
+* [`swarmctl agent permissions`↴](#swarmctl-agent-permissions)
+* [`swarmctl agent permissions get`↴](#swarmctl-agent-permissions-get)
+* [`swarmctl agent permissions set`↴](#swarmctl-agent-permissions-set)
+* [`swarmctl agent permissions clear`↴](#swarmctl-agent-permissions-clear)
 * [`swarmctl schedule`↴](#swarmctl-schedule)
 * [`swarmctl schedule add`↴](#swarmctl-schedule-add)
 * [`swarmctl schedule list`↴](#swarmctl-schedule-list)
@@ -66,6 +70,7 @@ Manage agents across the swarm
 * `create` — Queue creation of a new agent on a hive in this swarm
 * `mint-forge-token` — Check one agent's forge token, and mint it if it's missing or stale
 * `inference` — Show the model, effort, subagent models and subagent cap the swarm sets for an agent, or change the model, effort and subagent models
+* `permissions` — Show or change the capabilities the swarm grants an agent
 
 
 
@@ -198,6 +203,77 @@ Clear every field for an agent, so it goes back to its own values
 
 
 
+## `swarmctl agent permissions`
+
+Show or change the capabilities the swarm grants an agent.
+
+The hive running the agent reads them every 30 seconds and, when they change, restarts the agent after its current turn. Unset leaves the agent on its hive's default: none, except `ruth`, who gets `manage_root_agent`.
+
+**Usage:** `swarmctl agent permissions <COMMAND>`
+
+###### **Subcommands:**
+
+* `get` — Show the capabilities the swarm grants an agent, or that it sets none
+* `set` — Replace the capabilities the swarm grants an agent
+* `clear` — Clear the agent's permissions, so its hive's default applies again
+
+
+
+## `swarmctl agent permissions get`
+
+Show the capabilities the swarm grants an agent, or that it sets none
+
+**Usage:** `swarmctl agent permissions get [OPTIONS] <AGENT>`
+
+###### **Arguments:**
+
+* `<AGENT>` — Agent name
+
+###### **Options:**
+
+* `--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 agent permissions set`
+
+Replace the capabilities the swarm grants an agent
+
+**Usage:** `swarmctl agent permissions set [OPTIONS] --capabilities <CAPABILITIES> <AGENT>`
+
+###### **Arguments:**
+
+* `<AGENT>` — Agent name. A hive must declare it
+
+###### **Options:**
+
+* `--capabilities <CAPABILITIES>` — The capabilities to grant, comma-separated (`manage_root_agent`). `--capabilities ''` grants none
+* `--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 agent permissions clear`
+
+Clear the agent's permissions, so its hive's default applies again
+
+**Usage:** `swarmctl agent permissions clear [OPTIONS] <AGENT>`
+
+###### **Arguments:**
+
+* `<AGENT>` — Agent name
+
+###### **Options:**
+
+* `--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 schedule`
 
 Manage scheduled prompts.