refactor(#2916): drop the two obsolete startup migrations

Phase 4 (repoint every container onto `meta#<n>`) and phase 5 (rename
the `root` container to `h-root`) were marker-guarded one-shots for
layouts no live hive still has: containers are rendered onto `meta#<n>`
at creation, and the `h-` prefix has been the naming for far longer than
any deployment predates. A one-shot nobody can still trigger is dead
weight, so both are gone along with `repoint_container`,
`rename_manager_container`, `CONTAINER_TIMEOUT` and the two marker paths.

Phase 6 was not obsolete, only misplaced. Ruth's tool groups are now
seeded by `ensure_root_agent` on the one path that creates her, rather
than re-asserted on every hive-c0re boot. The skip-if-already-set guard
survives the move: a destroy+recreate under the same name must not reset
an operator's chosen group set back to MANAGER_DEFAULT.

That also settles a latent bug. Phase 4's marker check was a `return`,
not a skip, so on any hive carrying the marker phases 5 and 6 never ran
at all — the tool-group backfill, whose whole job was preventing a silent
privilege downgrade, has not executed here in a long time. Moving it to
create-time removes the question rather than answering it.

What stays is convergence: three unguarded, idempotent phases that re-run
each boot and no-op once their state is right. The module doc now names
the three categories so the next person can tell which kind they're
adding.

hive-c0re/src/paths.rs

@@ -248,18 +248,6 @@ pub fn matrix_register_token() -> PathBuf {
     state_root().join("matrix-register-token")
 }
 
-/// `.meta-migration-done` — one-shot marker: legacy meta layout migrated.
-#[must_use]
-pub fn meta_migration_marker() -> PathBuf {
-    state_root().join(".meta-migration-done")
-}
-
-/// `.hroot-rename-done` — one-shot marker: legacy hive-root rename applied.
-#[must_use]
-pub fn hroot_rename_marker() -> PathBuf {
-    state_root().join(".hroot-rename-done")
-}
-
 /// `/run/hyperhive` — the runtime root (host admin socket + per-agent dirs).
 #[must_use]
 pub fn runtime_root() -> PathBuf {