fix(dashboard): correct topology bulk-move endpoint description

The selection bar's M0V3 action uses two different endpoints:
- single agent: POST /api/topology/set-parent (form-encoded)
- multiple agents: POST /api/topology/set-parent-bulk (JSON array)

The previous text said 'POSTs to /api/topology/set-parent ... once
per selected agent' — wrong on both counts for multi-agent moves.
The bulk endpoint lands all moves in one topology.json commit.

docs/web-ui/dashboard.md

@@ -968,10 +968,15 @@ frosted-mauve bar slides up from the bottom of the viewport
     selection size. The dropdown lists every container that isn't IN
     the selection itself nor a descendant of any selected agent
     (client-side BFS cycle prevention across the whole batch; the
-    backend re-checks per-agent). On submit POSTs to
+    backend re-checks per-agent). On submit:
-    `/api/topology/set-parent` (form-encoded `child=<name>&new_parent=<target>`)
+    - **single agent** → `POST /api/topology/set-parent`
-    once per selected agent, which writes `topology.json` and re-emits
+      (form-encoded `child=<name>&new_parent=<target>`)
-    a container snapshot so the tree repaints without a page reload.
+    - **multiple agents** → `POST /api/topology/set-parent-bulk`
+      (JSON `[{ child, new_parent }]`; all moves land in a single
+      `topology.json` commit instead of one per agent)
+
+    Both write `topology.json` and re-emit a container snapshot so
+    the tree repaints without a page reload.
 - **`✕ clear`** button + `Esc` key clear the entire selection.
 
 Stale selections (agents destroyed while selected) are pruned on