feat: add a permission-gated UI action registry and action slots for plugins

Moves the terminal toolbar's hardwired AI button into a slot the ssh-terminal
plugin offers and the ai plugin fills, gated on ai.services.use.

src/ui/shell/ActionSlot.tsx

@@ -0,0 +1,82 @@
+import React from "react";
+import { useTranslation } from "react-i18next";
+import { useActionSlot } from "@/hooks/use-action-slot";
+import { invokeAction, type SlotContribution } from "@/shell/action-registry";
+
+interface ActionSlotProps {
+  slotId: string;
+  /**
+   * Arguments for the action, read at click time rather than on render. A
+   * getter keeps expensive context (a serialized terminal buffer, say) off the
+   * render path.
+   */
+  context?: () => unknown[];
+  /** Applied to the default button so a slot owner can match its own controls. */
+  className?: string;
+  /** Hides the whole slot. For a surface that is contextually unavailable. */
+  enabled?: boolean;
+  /** Renders a contribution the owner's own way instead of as a button. */
+  renderItem?: (
+    contribution: SlotContribution,
+    invoke: () => void,
+  ) => React.ReactNode;
+  /** Icon-only, for toolbars that collapse their labels. */
+  hideLabels?: boolean;
+}
+
+/**
+ * Renders whatever plugins have contributed to a named slot.
+ *
+ * Renders null when there is nothing to show, which covers both "no plugin
+ * contributed" and "the user may not see what was contributed". Those two
+ * cases are deliberately identical in the DOM: no placeholder, no disabled
+ * button, nothing hinting at a feature the user cannot reach.
+ */
+export function ActionSlot({
+  slotId,
+  context,
+  className,
+  enabled = true,
+  renderItem,
+  hideLabels,
+}: ActionSlotProps) {
+  const { t } = useTranslation();
+  const contributions = useActionSlot(slotId);
+
+  if (!enabled || contributions.length === 0) return null;
+
+  return (
+    <>
+      {contributions.map((contribution) => {
+        const invoke = () => {
+          void invokeAction(contribution.actionId, ...(context?.() ?? []));
+        };
+
+        if (renderItem) {
+          return (
+            <React.Fragment key={contribution.actionId}>
+              {renderItem(contribution, invoke)}
+            </React.Fragment>
+          );
+        }
+
+        const Icon = contribution.icon;
+        const label = t(contribution.titleKey);
+
+        return (
+          <button
+            key={contribution.actionId}
+            type="button"
+            className={className}
+            aria-label={label}
+            title={label}
+            onClick={invoke}
+          >
+            {Icon && <Icon className="size-4 shrink-0" />}
+            {!hideLabels && label}
+          </button>
+        );
+      })}
+    </>
+  );
+}