dashboard: convert the per-agent ⋮ context menu to a hive-agent-menu shadow-DOM component

Moves buildAgentMenu's DOM-building body, the menuItem/menuSep/menuLink
helpers, agentMenuPost, and the open-dropdown coordination logic out of
swarm.js and into a new <hive-agent-menu> autonomous custom element
(dashboard/src/agent-menu/), following the same shadow-DOM + one-dir-per-
component shape as hive-dialog. swarm.js's buildAgentMenu is now a thin
wrapper that constructs the element and sets ._opts before appending it,
same convention hive-dialog uses since a custom element created via
document.createElement can't take constructor args.

The module-level "one dropdown open at a time" singleton (previously a
single mutable variable in swarm.js) becomes a tracked Set of open
instances inside the component module; each instance closes itself via
its own close() method rather than another instance reaching into its
shadow internals. The document-level outside-click and Escape listeners
move into the component module too, keyed off e.composedPath() instead of
e.target.closest() -- shadow-DOM event retargeting means a plain
e.target check no longer reliably reaches into a specific instance's
shadow tree. closeAllAgentMenus() is exported for swarm.js's
buildAgentTree, which still needs to close any open menu before it
replaces the container tree DOM.

The hover-reveal opacity rule crosses the shadow boundary via a
--menu-btn-opacity custom property (custom properties inherit through
shadow boundaries): dashboard.css sets it on hover of the light-DOM
hive-agent-menu element, and the component sets it directly from JS while
its own dropdown is open, since that's component-internal state a CSS
selector out in the light DOM can't see. The host element itself takes on
the structural role (flex:none, position:relative, ...) the old
light-DOM .agent-menu wrapper div played, since its shadow tree's
absolute-positioned dropdown needs a positioned ancestor to anchor off of.

Verified end to end with a standalone esbuild-bundled test harness run
under headless chromium: row layout/flex sizing, hover-reveal opacity,
and dropdown positioning all render correctly, and a scripted interaction
pass (singleton exclusivity, outside-click close, Escape close, toggle
behavior, menu-item click close, and the exported closeAllAgentMenus())
all pass.

frontend/packages/dashboard/src/dashboard.css

@@ -279,78 +279,19 @@ body.dashboard-shell {
 }
 
 /* ── per-agent three-dot context menu ─────────────────────────────────────
-   Positioned after .card-body in the flex row. */
-.agent-menu {
-  flex: none;
-  position: relative;
-  align-self: flex-start;
-  margin-top: 0.3em;
-}
-.agent-menu-btn {
-  display: block;
-  background: none;
-  border: none;
-  color: var(--subtext0);
-  font-size: 1.1em;
-  line-height: 1;
-  cursor: pointer;
-  padding: 0.1em 0.4em;
-  border-radius: 4px;
-  opacity: 0;
-  transition: opacity 120ms, background 120ms, color 120ms;
-}
-.container-row:hover .agent-menu-btn,
-.agent-menu.open .agent-menu-btn {
-  opacity: 1;
-}
-.agent-menu-btn:hover,
-.agent-menu-btn:focus-visible {
-  background: color-mix(in srgb, var(--purple) 10%, transparent);
-  color: var(--purple);
-  outline: none;
-}
-.agent-menu-btn:focus-visible {
-  outline: 1px solid var(--purple);
-}
-.agent-menu-dropdown {
-  position: absolute;
-  right: 0;
-  top: calc(100% + 2px);
-  z-index: 50;
-  background: var(--bg-elev);
-  border: 1px solid var(--purple-dim);
-  border-radius: 6px;
-  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.4);
-  list-style: none;
-  margin: 0;
-  padding: 0.3em 0;
-  min-width: 10em;
-  white-space: nowrap;
-}
-.agent-menu-item {
-  display: block;
-  width: 100%;
-  background: none;
-  border: none;
-  color: var(--fg);
-  font-family: inherit;
-  font-size: 0.88em;
-  letter-spacing: 0.04em;
-  text-align: left;
-  padding: 0.4em 0.9em;
-  cursor: pointer;
-  text-decoration: none;
-  box-sizing: border-box;
-}
-.agent-menu-item:hover {
-  background: var(--border);
-  color: var(--purple);
-}
-.agent-menu-sep {
-  height: 1px;
-  background: var(--purple-dim);
-  margin: 0.3em 0;
-  padding: 0;
+   Positioned after .card-body in the flex row. The menu itself is the
+   <hive-agent-menu> shadow-DOM custom element (agent-menu/hive-agent-menu.js
+   + .css) — its own shadow-scoped stylesheet carries the button/dropdown/
+   item/separator rules and the `:host` rules that play the structural
+   role this light-DOM wrapper used to play. The one rule that still has
+   to live out here is the hover reveal: a light-DOM descendant-combinator
+   selector can't reach into the shadow tree, so it's relayed across the
+   shadow boundary via the `--menu-btn-opacity` custom property (custom
+   properties inherit through shadow boundaries) instead — the
+   component's own JS forces the same variable to 1 while its dropdown is
+   open (see hive-agent-menu.js's open()/close()). */
+.container-row:hover hive-agent-menu {
+  --menu-btn-opacity: 1;
 }
 /* Pending state splits queued vs running. */
 .container-row.pending .actions { opacity: 0.4; pointer-events: none; }