refactor: extract working-directory validation into testable module

- Add src/working-directory.ts with validateWorkingDirectory()
- Add __test__/working-directory.test.ts with 4 test cases
- Update main.ts to use validation function
- Throw clear error for non-existent or non-directory paths
- Rebuild dist/index.js

Addresses review feedback on #426

__test__/working-directory.test.ts

@@ -0,0 +1,43 @@
+import * as fs from 'fs'
+import * as os from 'os'
+import * as path from 'path'
+import {validateWorkingDirectory} from '../src/working-directory'
+
+describe('validateWorkingDirectory', () => {
+  let tmpDir: string
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'github-script-test-'))
+  })
+
+  afterEach(() => {
+    fs.rmSync(tmpDir, {recursive: true, force: true})
+  })
+
+  test('returns resolved path for existing directory', () => {
+    const result = validateWorkingDirectory(tmpDir)
+    expect(result).toBe(tmpDir)
+  })
+
+  test('resolves relative paths', () => {
+    const result = validateWorkingDirectory(tmpDir)
+    expect(path.isAbsolute(result)).toBe(true)
+  })
+
+  test('throws for non-existent directory', () => {
+    const nonExistent = path.join(tmpDir, 'does-not-exist')
+    expect(() => validateWorkingDirectory(nonExistent)).toThrow(
+      /does not exist/
+    )
+    expect(() => validateWorkingDirectory(nonExistent)).toThrow(nonExistent)
+  })
+
+  test('throws for file path instead of directory', () => {
+    const filePath = path.join(tmpDir, 'file.txt')
+    fs.writeFileSync(filePath, 'content')
+    expect(() => validateWorkingDirectory(filePath)).toThrow(
+      /is not a directory/
+    )
+    expect(() => validateWorkingDirectory(filePath)).toThrow(filePath)
+  })
+})