grant manage_root_agent by stored permission only, not by the name ruth

Permission::default_for and hive-sh4re's MANAGER_AGENT are gone: every
agent with no permissions key holds Permission::DEFAULT, ruth included.
The agent-name parameters that only fed default_for go with it
(permissions::effective, HiveCopy::before_read/after_read and their
callers).

swarm-controller seeds the grant ruth held by name: at start, a declared
ruth whose permissions are unset gets DEFAULT plus manage_root_agent
written into her key, once, logged. A set key is never touched. It is a
one-shot to delete once every swarm has passed it.

The swarm UI setup assistant opens the create form with
manage_root_agent checked, so a new ruth is created with the grant.
permissions_import no longer adds ruth to the import or splits her
default. Docs and help text no longer say ruth holds the grant by
default; swarmctl-cli.md regenerated.

frontend/packages/swarm-ui/src/shell/ruth-missing.ts

@@ -20,3 +20,7 @@ export async function ruthIsMissing(fetchImpl: Fetch): Promise<boolean> {
     return false;
   }
 }
+
+/** What the setup assistant's create form checks on top of the default set:
+ * ruth recovers the other agents, which takes `manage_root_agent`. */
+export const RUTH_EXTRA_PERMISSIONS = ["manage_root_agent"];