feat: add ESLint with error-handling rules

Add ESLint v9.35+ with typescript-eslint recommended config and
error-handling rules: preserve-caught-error (enforces { cause } when
re-throwing), no-unused-vars with caughtErrors:all, and
eslint-plugin-no-catch-all (warns on catch blocks that don't rethrow).

Fix existing violations: add error cause to container-runtime rethrow,
prefix unused vars with underscore, remove unused imports.

https://claude.ai/code/session_01JPjzhBp9PR5LtfLWVDrYrH

src/sender-allowlist.test.ts

@@ -1,7 +1,7 @@
 import fs from 'fs';
 import os from 'os';
 import path from 'path';
-import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 
 import {
   isSenderAllowed,