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).

packages/integrations/package.json

@@ -13,8 +13,8 @@
   },
   "exports": {
     ".": "./index.ts",
+    "./response-contract": "./src/base/response-contract.ts",
     "./test-connection": "./src/base/test-connection/index.ts",
-    "./client": "./src/client.ts",
     "./types": "./src/types.ts"
   },
   "scripts": {