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/index.ts

@@ -740,6 +740,7 @@ export type {
   ResearchCancellationState,
 } from "./research-types.js";
 
+export { isExperimentalFeatureEnabled } from "./experimental-features.js";
 export { isResearchExperimentalEnabled, resolveResearchSettings } from "./research-settings.js";
 export type { ResolvedResearchSettings } from "./research-settings.js";
 export { resolveEvalSettings } from "./eval-settings.js";