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/getting-started/setup.md

@@ -93,8 +93,10 @@ SSO login asks for its forge password once, to link the two.
 
 hive-c0re creates no agent at startup. You create ruth, the manager agent,
 like any other: open the swarm UI start page and click the paperclip, which
-opens the create form with `ruth` filled in. The form declares her, creates
-her config repo and deploys her to the hive you pick.
+opens the create form with `ruth` filled in and the `manage_root_agent`
+permission checked: that permission mounts the other agents into her
+container so she can recover them. The form declares her, creates her
+config repo and deploys her to the hive you pick.
 
 The swarm declares a new agent `paused`, so without a further step ruth runs
 but drives no turns. Open `/agents` in the swarm UI and set her `up` from her