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.

docs/agent-lifecycle/persistence.md

@@ -408,7 +408,8 @@ Contents:
   to live here; nothing reads or writes it now. An old copy in the meta repo
   is harmless and you can delete it.
 
-The root agent has the meta dir RO-mounted at `/meta/`.
+An agent holding `manage_root_agent`, normally ruth, has the meta dir
+RO-mounted at `/meta/`.
 
 Outside the meta repo, `/var/lib/hyperhive/swarm-agent-permissions.json`
 holds the permissions the swarm grants this hive's agents, as