Add explicit report output files

[kanuwrld]

What changed

• add --output PATH while preserving stdout as default;
• write UTF-8 reports with exactly one trailing newline;
• surface missing parent and write failures as exit code 2;
• expose destination through composite Action input;
• document behavior and add regression tests.

Why

Direct file output works consistently across shells and operating systems.
Automation no longer needs fragile redirection.

Closes #5.

Pairing

Implementation created by Codex Agent from requirements and product decisions
co-authored by @kanuwrld. Commit contains GitHub-recognized
Co-authored-by metadata.

Validation

• python -m unittest discover -s tests -v — 15 passed
• python -m compileall -q src tests
• generated JSON file parsed with python -m json.tool
• git diff --check

Risk

Low. Default stdout behavior and existing formats remain unchanged.