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>

utils/postCleanup.ts

@@ -1,6 +1,6 @@
 import { LEAPING_INTO_ACTION_PREFIX } from "../mcp/comment.ts";
+import { buildPullfrogFooter } from "./buildPullfrogFooter.ts";
 import { log } from "./cli.ts";
-import { buildErrorCommentBody } from "./exitHandler.ts";
 import { createOctokit, parseRepoContext } from "./github.ts";
 import { type ResolvedPromptInput, resolvePromptInput } from "./payload.ts";
 import { getJobToken } from "./token.ts";
@@ -14,6 +14,30 @@ type JsonPromptInput = Extract<ResolvedPromptInput, object>; // not string
  * */
 const SHOULD_CHECK_REASON = true;
 
+/**
+ * Build error comment body with error message and footer
+ */
+function buildErrorCommentBody(params: {
+  owner: string;
+  repo: string;
+  runId: string | undefined;
+  isCancellation: boolean;
+}): string {
+  const workflowRunLink = params.runId
+    ? `[workflow run logs](https://github.com/${params.owner}/${params.repo}/actions/runs/${params.runId})`
+    : "workflow run logs";
+  const errorMessage = params.isCancellation
+    ? `This run was cancelled 🛑\n\nThe workflow was cancelled before completion. Please check the ${workflowRunLink} for details.`
+    : `This run croaked 😵\n\nThe workflow encountered an error before any progress could be reported. Please check the ${workflowRunLink} for details.`;
+  const footer = buildPullfrogFooter({
+    triggeredBy: true,
+    workflowRun: params.runId
+      ? { owner: params.owner, repo: params.repo, runId: params.runId }
+      : undefined,
+  });
+  return `${errorMessage}${footer}`;
+}
+
 /**
  * Validate that the progress comment is stuck on "Leaping into action"
  * Fetches the comment by ID and checks if it starts with LEAPING_INTO_ACTION_PREFIX