fix(988): hide inapplicable actions in agent context menu

Per mara feedback: don't show disabled actions at all.
- restart/stop only rendered when agent is running
- start only rendered when agent is stopped
- removed .agent-menu-item:disabled CSS rule (no longer needed)

Also switch Escape handler to stopImmediatePropagation so the
selection-clear keydown listener doesn't co-fire when closing a menu.

frontend/packages/dashboard/src/dashboard.css

@@ -384,13 +384,9 @@ a:hover {
   padding: 0.4em 0.9em;
   cursor: pointer;
 }
-.agent-menu-item:not(:disabled):hover {
+.agent-menu-item:hover {
   background: var(--surface1);
 }
-.agent-menu-item:disabled {
-  opacity: 0.35;
-  cursor: default;
-}
 .agent-menu-sep {
   height: 1px;
   background: var(--surface2);