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/swarm_notices.rs

@@ -1,10 +1,8 @@
 //! Publishing lifecycle notices onto the swarm queue.
 //!
-//! Replaces the old `push_todo(MANAGER_AGENT, ...)` fallback the
-//! lifecycle-notice call sites used to reach for when there was nobody
-//! else to tell. Every hive is swarm-controlled now, so there is no case
-//! left that needs a manager-agent recipient — this module has no such
-//! fallback, on purpose, not by omission.
+//! There is no fallback recipient agent: every hive is swarm-controlled,
+//! so a notice always goes to the swarm queue, and no agent receives it by
+//! name.
 //!
 //! **Why a stream and not the [`crate::swarm_status`] KV bucket shape**:
 //! a status snapshot has a current value a late reader can always ask