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.

hive-c0re/src/migrate.rs

@@ -11,10 +11,9 @@
 //!   only if it cannot be expressed as convergence, and expect to delete
 //!   it once every hive has passed it.
 //! - **Create-time setup.** A thing that is true from birth is set where
-//!   the agent is created, not re-asserted each morning. Ruth's
-//!   permissions need neither: with no key in the swarm's
-//!   `agent-permissions` bucket she holds the code-constant default
-//!   (`Permission::default_for`).
+//!   the agent is created, not re-asserted each morning: ruth's
+//!   `manage_root_agent`, for one, is stored in the swarm's
+//!   `agent-permissions` bucket when the swarm UI creates her.
 //!
 //! Kill-switch: `HIVE_SKIP_META_MIGRATION=1`. Full sequence and phase
 //! details: `docs/agent-lifecycle/config-changes.md::Startup migrations (older hosts)`.