Improve test structure and mocking approach

[threeal]

Summary

• Replace process.env.INPUT_PATH hacks with vi.mock("gha-utils") to properly mock getInput at the module level
• Use a local .main.test.tmp directory (in src/) instead of os.tmpdir(), scoped to the test file
• Reset process.exitCode in beforeEach instead of manually at the end of one test
• Use named imports from node:fs/promises in both main.ts and main.test.ts

Test plan

• All tests pass (pnpm test)
• Pre-commit hook passes (lefthook run pre-commit --all-files)

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