feat: port release-v2 experience improvements from PR #6582

Brings the semantically-correct unification work from the closed
agent/release-v2-experience PR into release/v2:

- universal create surface (header, board, spotlight) with permission-aware actions
- board empty state and setup checklist with widget/connection recipes
- in-widget integration editing via inspector modal, preserving unsaved widget settings
- single-source widget-integration capability map (nativeFeatureCapabilities)
- multi-endpoint Docker singleton with per-endpoint failure isolation, DOCKER_ENDPOINTS descriptors and service reconciliation
- Kubernetes multi-context registry with availability probing and context selector
- response-contract test harness and setup analytics property boundaries

Excludes the feature-platform generator/workbench tooling (deferred).

apps/nextjs/src/components/board/board-setup-checklist.module.css

@@ -0,0 +1,21 @@
+.root {
+  position: absolute;
+  z-index: 4;
+  bottom: calc(var(--mantine-spacing-md) * var(--board-canvas-inverse-scale, 1));
+  left: calc(var(--mantine-spacing-md) * var(--board-canvas-inverse-scale, 1));
+  max-width: min(24rem, calc(100% - 2 * var(--mantine-spacing-md)));
+  pointer-events: auto;
+}
+
+.panel {
+  width: min(24rem, calc(100vw - 2rem));
+}
+
+.step {
+  border-radius: var(--mantine-radius-md);
+  padding: var(--mantine-spacing-xs);
+}
+
+.step[data-complete] {
+  background: light-dark(var(--mantine-color-green-0), rgba(47, 158, 68, 0.1));
+}