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.

frontend/packages/dashboard/src/core.js

@@ -96,7 +96,7 @@ function renderTombstones(s) {
 }
 
 // ─── stale permission entries (K3PT ST4T3 pane sub-section) ──────────────
-// Agents with explicit capability / tool-group JSON entries but no live
+// Agents with explicit tool-group JSON entries but no live
 // container AND no kept-state tombstone (e.g. renamed agents like the old
 // "root" manager name). Ghost detection is server-side via
 // GET /api/permissions/stale so the client doesn't need to maintain a
@@ -114,7 +114,7 @@ function renderStalePerms(root, ghosts) {
     el(
       "p",
       { class: "meta" },
-      "agents with explicit capability or tool-group entries but no live container " +
+      "agents with explicit tool-group entries but no live container " +
         "or kept state (typically renamed or manually-deleted agents whose JSON entries persisted).",
     ),
   );
@@ -129,7 +129,7 @@ function renderStalePerms(root, ghosts) {
       {
         type: "button",
         class: "btn btn-destroy",
-        title: "remove explicit capability and tool-group entries for " + name,
+        title: "remove explicit tool-group entries for " + name,
       },
       "✕ clear perms",
     );
@@ -194,9 +194,6 @@ const SSE_HANDLERS = {
   },
   // Refresh the stale-perms sub-section when perm data changes (a ghost
   // was cleared, or perms were saved for an agent whose name collides).
-  capabilities_changed(_ev) {
-    if (stalePermsLoaded) fetchAndRenderStalePerms();
-  },
   tool_groups_changed(_ev) {
     if (stalePermsLoaded) fetchAndRenderStalePerms();
   },
@@ -234,8 +231,7 @@ async function init() {
   // (subscription discipline, part 1 of the dashboard-event-stream-
   // split issue).
   const es = openStream(
-    "/api/dashboard/stream?kinds=tombstones_changed," +
-      "capabilities_changed,tool_groups_changed",
+    "/api/dashboard/stream?kinds=tombstones_changed,tool_groups_changed",
   );
   if (es) {
     es.onmessage = (e) => {