fix(setup): register explicit MCP runtimes and preserve launcher identity

src/process/CurrentEntrypointPath.ts

@@ -1,13 +1,21 @@
 import { resolve } from "node:path";
+import {
+  javaScriptRuntime,
+  type JavaScriptRuntime,
+} from "../domain/runtimeVersion.js";
+import { absoluteCallerPath } from "../filesystem/callerPath.js";
 
-/** Node's absolute argv script selection, or undefined when none was supplied.
+/** Runtime-specific absolute argv script selection, or undefined when none was supplied.
  * Native executable paths have different traversal semantics and must not use this.
  */
 export const normalizeEntrypointPath = (
   entrypoint: string | undefined,
+  runtime: JavaScriptRuntime["id"] = javaScriptRuntime(process.versions).id,
 ): string | undefined =>
   entrypoint === undefined || entrypoint.length === 0
     ? undefined
-    : resolve(entrypoint);
+    : runtime === "bun"
+      ? absoluteCallerPath(entrypoint)
+      : resolve(entrypoint);
 
 export const CURRENT_ENTRYPOINT_PATH = normalizeEntrypointPath(process.argv[1]);