refactor(hivectl): move quota + subvol under agents

Both groups only ever act on a single managed agent's state dir, so
they belong in the `agents` namespace rather than as top-level verbs
next to `forge` / `matrix` / `wg`.

Renames `quota limit` -> `quota set`: the enclosing group already
carries the noun, so the bare verb matches the flat `set-parent` /
`set-limits` spelling without stuttering, and it removes the
`set-limits` (cpu/mem) vs `quota limit` (disk) ambiguity. Adds a
cross-pointer from `set-limits` to `agents quota`.

Handlers stay in their own modules; `run_agents` gains the reparenting
glue. Regenerates docs/tools/hivectl-cli.md.

Refs #2724

hivectl/src/quota.rs

@@ -1,4 +1,4 @@
-//! `hivectl quota` — per-agent disk accounting + optional quotas via
+//! `hivectl agents quota` — per-agent disk accounting + optional quotas via
 //! btrfs qgroups. The daemon holds the privileged helper that reads /
 //! sets qgroups; hivectl relays the request and formats the reply.
 
@@ -26,7 +26,7 @@ pub(crate) async fn quota_show(socket: &Path, name: Option<&str>) -> Result<()>
     .await
     .with_context(|| format!("connect to daemon socket {}", socket.display()))?;
     if !resp.ok {
-        // Carries the "btrfs quota not enabled — run `hivectl quota enable`
+        // Carries the "btrfs quota not enabled — run `hivectl agents quota enable`
         // first" hint when qgroups are off.
         bail!("{}", resp.error.as_deref().unwrap_or("quota show failed"));
     }