frontend: split hive-agent-menu's generic dropdown mechanics into a shared hive-menu component

<hive-agent-menu> bundled two concerns: the agent-specific trigger/item
list, and generic "click a trigger, get a positioned dropdown" mechanics
(shadow attach, open/close, singleton close-on-open coordination,
outside-click/Escape handling). Pulled the latter out into a new
@hive/shared/hive-menu.js (<hive-menu>), following the established
per-component-directory + ._opts-before-append shadow-DOM pattern
(<hive-dialog>). <hive-agent-menu> now just builds the "⋮" trigger and
the action list and hands them to an internal <hive-menu> instance.

<hive-menu> takes ownership of every <hive-menu> instance in the app for
singleton coordination (closeAllMenus, renamed from closeAllAgentMenus)
— a deliberate widening from the old per-agent-menu-only tracking, since
the mechanism was never agent-specific to begin with.

The one subtlety worth spelling out: <hive-menu> projects the caller's
opaque trigger/content nodes via named <slot>s rather than moving them
into its own shadow root. That's load-bearing, not cosmetic — if it
re-parented them into its own shadow tree instead, <hive-agent-menu>'s
own classes (.agent-menu-btn, .agent-menu-item, ...) would stop applying,
since a <style> only styles elements within the same shadow tree/document
it's part of, and only slotting (not re-parenting) keeps the caller's
nodes in the caller's own tree for styling purposes. That in turn made
<hive-agent-menu>'s own shadow root redundant once it wasn't the thing
positioning or owning open/close state anymore, so it's dropped in favor
of a plain light-DOM element styled by dashboard.css (already the one
page it renders on) — hive-agent-menu.css is gone, its rules folded into
dashboard.css's per-agent-menu section, minus the positioning rules that
moved into hive-menu.css as the new generic `.menu-dropdown` wrapper.

Verified with a standalone esbuild bundle + a cached nix chromium driven
over raw CDP (no puppeteer/playwright/python3 available): hover-reveal
opacity, dropdown open/close/positioning, outside-click/Escape dismissal,
and cross-instance singleton coordination all behave identically to
before the split.

frontend/packages/dashboard/src/dashboard.css

@@ -279,17 +279,90 @@ body.dashboard-shell {
 }
 
 /* ── per-agent three-dot context menu ─────────────────────────────────────
-   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()). */
+   Positioned after .card-body in the flex row. `<hive-agent-menu>`
+   (agent-menu/hive-agent-menu.js) is a light-DOM element with no shadow
+   root of its own — the generic dropdown mechanics (shadow attach,
+   open/close, positioning) live in the shared `<hive-menu>` component
+   instead (@hive/shared/hive-menu.js), which slots this element's
+   trigger/content nodes into position rather than moving them into its
+   own shadow root. That keeps these nodes in the ordinary light DOM the
+   whole way down, so this ordinary global stylesheet reaches them
+   directly — see hive-agent-menu.js's header for the full reasoning.
+   `hive-agent-menu` itself just needs the flex/alignment role the old
+   light-DOM `.agent-menu` wrapper div played in this row; the
+   `position: relative` an absolutely-positioned dropdown needs now lives
+   on `<hive-menu>`'s own `:host` instead, since that's the element whose
+   shadow tree the dropdown is actually positioned within. */
+hive-agent-menu {
+  flex: none;
+  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: var(--menu-btn-opacity, 0);
+  transition: opacity 120ms, background 120ms, color 120ms;
+}
+.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);
+}
+/* Positioning (`position`/`top`/`right`/`z-index`) is generic and lives
+   on `<hive-menu>`'s own `.menu-dropdown` wrapper instead — this is just
+   the item list's visual chrome. */
+.agent-menu-dropdown {
+  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;
+}
+/* Hover reveal: `--menu-btn-opacity` is forced to 1 while a hover
+   selector (or the trigger's own open state, set by <hive-menu>) applies
+   — inherits down through `hive-agent-menu` → `<hive-menu>` → the
+   trigger button same as any custom property, shadow trees included. */
 .container-row:hover hive-agent-menu {
   --menu-btn-opacity: 1;
 }