feat(ui/alerts): MustacheEditor component (CM6 shell with completion + linter)

Wires the mustache-completion source and mustache-linter into a CodeMirror 6
EditorView. Accepts kind (filters variables) and reducedContext (env-only for
connection URLs). singleLine prevents newlines for URL/header fields. Host
ref syncs when the parent replaces value (promotion prefill).

ui/src/components/MustacheEditor/MustacheEditor.test.tsx

@@ -0,0 +1,34 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { MustacheEditor } from './MustacheEditor';
+
+describe('MustacheEditor', () => {
+  it('renders the initial value', () => {
+    render(
+      <MustacheEditor
+        value="Hello {{rule.name}}"
+        onChange={() => {}}
+        kind="ROUTE_METRIC"
+        label="Title template"
+      />,
+    );
+    expect(screen.getByText(/Hello/)).toBeInTheDocument();
+  });
+
+  it('renders a textbox and does not call onChange before user interaction', () => {
+    const onChange = vi.fn();
+    render(
+      <MustacheEditor
+        value=""
+        onChange={onChange}
+        kind="ROUTE_METRIC"
+        label="Title template"
+      />,
+    );
+    const editor = screen.getByRole('textbox', { name: 'Title template' });
+    expect(editor).toBeInTheDocument();
+    // CM6 fires onChange via transactions, not DOM input events; without a real
+    // user interaction the callback must remain untouched.
+    expect(onChange).not.toHaveBeenCalled();
+  });
+});