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/lifecycle/host_config.rs

@@ -843,13 +843,13 @@ mod tests {
         assert_eq!(std::fs::read(&path).expect("read"), truncated.as_bytes());
     }
 
-    /// Unset is the default, which gives ruth the recovery mounts and nobody
-    /// else; a grant or an explicit empty list replaces it.
+    /// Unset is the default, which gives nobody the recovery mounts, `ruth`
+    /// included; only a grant does.
     #[test]
-    fn the_swarm_grant_or_the_default_decides_the_recovery_mounts() {
+    fn the_swarm_grant_decides_the_recovery_mounts() {
         let dir = tempfile::tempdir().expect("tempdir");
         let path = dir.path().join("swarm-agent-permissions.json");
-        assert!(holds_manage_root_agent("ruth", &path));
+        assert!(!holds_manage_root_agent("ruth", &path));
         assert!(!holds_manage_root_agent("alice", &path));
         std::fs::write(
             &path,