fix: proper os.Stat error handling in fileExists (avoid nil deref). Fix #9

[icy]

Contents below generated by Copilot (gpt-5-mini)

1. The MR is completely created by AI. Test cases fixed by @icy
2. Problem: fileExists previously ignored os.Stat errors except for os.IsNotExist(err) and then dereferenced info.IsDir() even when info was nil, which causes a nil-pointer dereference when os.Stat returns a non-nil error that is not ErrNotExist (permission errors, transient FS errors, etc.).
3. Fix: fileExists now returns (bool, error). For any os.Stat error (including file-not-exist) the function returns false and a single combined error message ("file non-exist or error: ..."). The caller in main checks the returned error first and fails with a clear message. This avoids dereferencing a nil FileInfo and makes the behavior explicit.
4. Rationale: combining all os.Stat errors into a single case matches the user's request ("For any error, return file non-exist or error (combine all error into a single case)"), simplifies caller logic, and prevents runtime panics.
5. Tests / next steps: we recommend adding unit tests that simulate os.Stat returning ErrNotExist and permission errors to prevent regressions. Optionally change fileExists to return richer typed errors if finer-grained handling is desired later.