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.

hive-agent/src/mcp_config.rs

@@ -49,8 +49,9 @@ fn allowed_capability_tools() -> Vec<String> {
         let t = token.trim().to_ascii_lowercase();
         match t.as_str() {
             "read_host_journal" => tools.push("get_host_journal".to_owned()),
-            // manage_root_agent doesn't expose a new MCP tool: it gates
-            // existing lifecycle tools via topology enforcement instead.
+            // manage_root_agent exposes no MCP tool at all: it is a
+            // host-side grant, enforced by hive-c0re when it computes the
+            // container's bind mounts. Nothing for the harness to register.
             "manage_root_agent" => {}
             unknown => {
                 tracing::warn!(capability = %unknown, "unrecognised capability in HIVE_CAPABILITIES — skipped");