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.

hive-c0re/src/dashboard/mod.rs

@@ -37,7 +37,7 @@ use crate::lifecycle;
         (name = "lifecycle_ops", description = "agent container lifecycle: rebuild/restart/start/stop/pause/limits"),
         (name = "meta_inputs", description = "bulk flake-input update for the meta flake"),
         (name = "misc_api", description = "mark-all-read, compose, spawn-request, hive stats"),
-        (name = "permissions", description = "tool-group + capability assignment for agents"),
+        (name = "permissions", description = "tool-group assignment for agents"),
         (name = "rebuild_queue", description = "rebuild-queue cancel"),
         (name = "state_files", description = "proxied reads of allow-listed per-agent state files"),
         (name = "state_snapshot", description = "cold-load dashboard snapshot"),
@@ -126,8 +126,6 @@ fn app(coord: Arc<Coordinator>) -> axum::Router {
         .routes(routes!(build_logs::get_build_log_raw))
         .routes(routes!(permissions::get_tool_groups))
         .routes(routes!(permissions::post_tool_groups))
-        .routes(routes!(permissions::get_capabilities))
-        .routes(routes!(permissions::post_capabilities))
         .routes(routes!(permissions::post_permissions))
         .routes(routes!(permissions::get_stale_permissions))
         .routes(routes!(permissions::delete_agent_permissions))
@@ -380,8 +378,6 @@ mod router_build_probe {
             .routes(routes!(build_logs::get_build_log_raw))
             .routes(routes!(permissions::get_tool_groups))
             .routes(routes!(permissions::post_tool_groups))
-            .routes(routes!(permissions::get_capabilities))
-            .routes(routes!(permissions::post_capabilities))
             .routes(routes!(permissions::post_permissions))
             .routes(routes!(permissions::get_stale_permissions))
             .routes(routes!(permissions::delete_agent_permissions))