`roles.json` had a seeding path: `topology::reconcile_roles` granted
`can_manage_top_level_agents` to MANAGER_NAME on every meta sync, and that
role is what put every agent's state/config dir, `/applied` and `/meta` into
the root agent's nspawn binds. Collapsing the role into the capability left
the capability store with no equivalent, so the root agent would have lost
its recovery mounts at its next container rebuild — silently, and only then,
since nspawn bakes bind flags at container start.
Seed it in `ensure_root_agent`, before the create/start branch so a hive
whose root container already exists still gets the grant. Seeded once rather
than re-ensured every boot: the role kept an empty-list tombstone so a revoke
stuck, and the capability store deletes an emptied entry instead, so "no
entry for the manager" cannot tell a fresh hive from a deliberate revoke.
File existence can — every grant and revoke writes capabilities.json, even
when the result is `{}` — so the seed fires only while the store has never
been written, and is inert forever after.
The target is `lifecycle::MANAGER_NAME`, the same const the deleted role seed
used and the one definition of "the root agent" in the tree; no name literal
at the seeding site. The written string comes from
`Capability::ManageRootAgent` via `IntoStaticStr` rather than being spelled
out, so it cannot drift into a name `prune_unknown` would drop.
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