fix: replace domain-specific exit handler with generic signal handler registry (#299)

* fix: replace domain-specific exit handler with generic signal handler registry

Rewrite exitHandler.ts as a generic, domain-agnostic exit signal module
that exports onExitSignal(handler) returning a dispose function.

- subprocess.ts now registers via onExitSignal instead of direct
  process.on(SIGINT/SIGTERM) calls
- resolveTokens registers a signal handler that captures tokens by
  closure, fixing the race condition where the exit handler would
  read the wrong token after disposal
- Remove setupExitHandler and runCleanup — domain cleanup is handled
  by post.ts + await using

Co-authored-by: Cursor <cursoragent@cursor.com>

* tweaks

* simplify handler installation

* extract to util

* fix race in dispose

* wrap dispose body in try/finally to ensure disposingRef always settles

---------

Co-authored-by: Mateusz Burzyński <mateuszburzynski@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: pullfrog[bot] <226033991+pullfrog[bot]@users.noreply.github.com>

entry.ts

@@ -6,7 +6,6 @@
 
 import * as core from "@actions/core";
 import { main } from "./main.ts";
-import { runCleanup } from "./utils/exitHandler.ts";
 
 async function run(): Promise<void> {
   try {
@@ -22,8 +21,6 @@ async function run(): Promise<void> {
   } catch (error) {
     const errorMessage = error instanceof Error ? error.message : "Unknown error occurred";
     core.setFailed(`Action failed: ${errorMessage}`);
-  } finally {
-    await runCleanup();
   }
 }