feat(FN-3731): add action-gate reload endpoint for reloadable exempt-tools

Adds a reload endpoint for action-gate exempt tools, exposing `POST /api/system/reload-exempt-tools` on the dashboard to hot-reload the engine's mutable EXEMPT_TOOLS registry without a restart. The endpoint is wired through `agent-action-gate.ts` with test coverage for reloadable behavior.

Fusion-Task-Id: FN-3731

packages/engine/src/index.ts

@@ -1,4 +1,5 @@
 export { AgentLogger, type AgentLoggerOptions, summarizeToolArgs } from "./agent-logger.js";
+export { reloadExemptTools, addToExemptTools, getExemptToolNames } from "./agent-action-gate.js";
 export {
   createTaskCreateTool,
   createTaskDocumentReadTool,