feat(FN-3706): gate research tools behind experimental feature

The merge introduces a research tool surface gating mechanism: shared availability helpers in core and engine, applied to the executor and triage agent so research prompts and tool exposure are gated behind experimental-feature flags, with tests covering the new logic.

Fusion-Task-Id: FN-3706

packages/core/src/experimental-features.ts

@@ -0,0 +1,24 @@
+import type { Settings } from "./types.js";
+
+const LEGACY_EXPERIMENTAL_FEATURE_ALIASES: Record<string, string> = {
+  devServer: "devServerView",
+};
+
+export function isExperimentalFeatureEnabled(
+  settings: Pick<Settings, "experimentalFeatures"> | undefined,
+  key: string,
+): boolean {
+  const features = settings?.experimentalFeatures;
+  if (!features) return false;
+
+  const canonicalKey = LEGACY_EXPERIMENTAL_FEATURE_ALIASES[key] ?? key;
+  if (features[canonicalKey] === true) return true;
+
+  for (const [legacyKey, aliasCanonical] of Object.entries(LEGACY_EXPERIMENTAL_FEATURE_ALIASES)) {
+    if (aliasCanonical === canonicalKey && features[legacyKey] === true) {
+      return true;
+    }
+  }
+
+  return false;
+}