fix(dashboard): drop stale manager destroy-hiding comment + vestigial block

Follow-up to the destroy-guard removal — the bootstrap/root container is
now destroyable end-to-end. The frontend already offered DESTR0Y/PURG3
for every container; the only manager-gating left was a stale doc-comment
('destroy/purge hidden for the manager') and a vestigial bare block
wrapping the menu appends. Drop both (no behaviour change), and update the
dashboard.md menu doc: 'disabled for the bootstrap container' -> available
for any container (hive-c0re recreates it on next startup if destroyed).

docs/web-ui/dashboard.md

@@ -925,7 +925,7 @@ frosted-mauve bar slides up from the bottom of the viewport
   - `■ ST0P` — running agents only
   - `▶ ST4RT` — stopped agents only
   - `↻ R3BU1LD` — always available
-  - `DESTR0Y` / `PURG3` — sub-agents only (disabled if the root/bootstrap container selected)
+  - `DESTR0Y` / `PURG3` — always available
   - `⇡ M0V3 → ROOT` — promote selected agents to top-level
     (parent = null); disabled when all selected are already at root.
     Backend `topology::set_parent` refuses moves it can't satisfy