dashboard: selection-bar M0V3 affordances for re-parenting agents (#486)

Backend POST /api/topology/set-parent already shipped; the dashboard
was missing the operator surface to drive it. Adds two affordances
to the SW4RM tab's selection bar (alongside the existing R3ST4RT /
ST0P / ST4RT / R3BU1LD / DESTR0Y / PURG3 actions):

- '⇡ M0V3 → ROOT' (bulk): promote selected agents to top-level
  (parent=null). Disabled when all selected are already at root or
  the selection includes the manager (backend refuses anyway).

- '⇢ M0V3 → [pick]' (single-agent only): inline <select> dropdown
  + button pair. Dropdown lists every container that isn't the
  target nor a descendant of it (client-side BFS via the existing
  c.parent map). On submit POSTs form-encoded
  'child=<name>&new_parent=<target>' to /api/topology/set-parent;
  the backend re-checks the cycle invariant and re-emits a
  container snapshot so the tree repaints without a reload.

Both POSTs hit a single URL, so addBulkButton grew an optional
'perAgentBodyFor(name)' hook to handle the body-driven endpoint
shape (vs the URL-suffix /start/<name> shape every other action
uses). Lifecycle endpoints unchanged.

Mauve chrome (var(--mauve)) reads as 'structural change' rather
than the destructive red / amber of destroy / rebuild.

closes #486

docs/web-ui.md

@@ -440,6 +440,16 @@ frosted-mauve bar slides up from the bottom of the viewport
   - `▶ ST4RT` — stopped agents only
   - `↻ R3BU1LD` — always available
   - `DESTR0Y` / `PURG3` — sub-agents only (disabled if manager selected)
+  - `⇡ M0V3 → ROOT` (#486) — promote selected agents to top-level
+    (parent = null); disabled when all selected are already at root
+    or when the selection includes the manager.
+  - `⇢ M0V3 → [select]` (#486) — single-agent-only inline picker;
+    the dropdown lists every container that isn't the target itself
+    nor a descendant of it (client-side cycle prevention; the
+    backend's `topology::set_parent` re-checks). On submit POSTs to
+    `/api/topology/set-parent` (form-encoded `child=<name>&new_parent=<target>`),
+    which writes `topology.json` and re-emits 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