collapse the roles.json mount grant into the ManageRootAgent capability

The hive had two spellings of "this agent may act on agents that aren't
its children": the `ManageRootAgent` capability, which nothing checked,
and a `can_manage_top_level_agents` role in a third meta store,
`roles.json`, which owned the real grant — the bind mounts that put
another agent's state (rw) and config (ro) inside the holder's
container. The two drifted independently, and with the parent/child
hierarchy removed the role's set (`parent.is_none()`) silently became
every agent while nothing said so.

Collapse them. The mount grant now hangs off
`Capability::ManageRootAgent`, looked up through the one capability
path that already exists (`capabilities::has_cap` over
`capabilities.json`) rather than a second mechanism. `roles.json` and
everything that read, wrote or reconciled it is gone, along with its
`meta.rs` staging and commit-label wiring; nothing in the tree reads
that file any more.

The enum variant keeps its name deliberately. Renaming it would turn
every `manage_root_agent` already stored in `capabilities.json` into an
unrecognised name that `prune_unknown` drops without asking. Its
meaning, not its spelling, is what changed: "may manage any agent". The
doc comment and the description string now say that.

`top_level_agents()`/`top_level_agents_in()` are replaced by
`all_agents()`/`all_agents_in()`. Under "manage any agent" the mounted
set is every agent by definition, so the code states it instead of
deriving it from a predicate that no longer discriminates — and the
call-site comment explains that, because it otherwise reads as a
widening. The holder is no longer bound as its own virtual child: that
reproduced the own-state and own-config mounts exactly, so dropping it
loses nothing.

docs/web-ui/dashboard.md

@@ -440,7 +440,7 @@ The current capabilities are:
 
 | Name | Effect |
 |------|--------|
-| `manage_root_agent` | allows the `set_status` / lifecycle tools on the root agent |
+| `manage_root_agent` | mounts every agent's state (rw) + config (ro) into this agent's container so it can manage/recover any agent |
 | `read_host_journal` | unlocks `get_host_journal` to read journald from inside a container |
 
 Each row is one agent. Columns are the capability names returned by