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

hive-c0re/src/server.rs

@@ -603,7 +603,7 @@ async fn handle_quota_show(name: Option<&str>) -> Result<HostResponse> {
                 // varies across btrfs-progs versions).
                 if msg.to_ascii_lowercase().contains("quota not enabled") {
                     return Ok(HostResponse::error(
-                        "btrfs quota not enabled — run `hivectl quota enable` first",
+                        "btrfs quota not enabled — run `hivectl agents quota enable` first",
                     ));
                 }
                 // A plain-dir agent (no subvolume) has no qgroup; note it