Preserve oxlint GitHub reporter output in `vp check` on GitHub Actions

## Context

PR #914 fixed `vp check` failing on GitHub Actions by forcing `--format=default` for oxlint. The root cause was that when `GITHUB_ACTIONS=true`, oxlint auto-switches to the GitHub reporter, which omits the human-readable summary output that `vp check` parses to determine pass/fail status.

While the fix resolves the immediate breakage, it loses the advantage of oxlint's customized GitHub Actions reporter output (e.g., inline annotations on PR diffs).

## Problem

Currently `vp check` parses oxlint's human-readable summary output to produce unified pass/fail lines. This is incompatible with oxlint's GitHub reporter format, forcing us to disable it.

## Desired Behavior

When running on GitHub Actions, `vp check` should:
1. Still produce correct pass/fail output (not break as before)
2. Preserve oxlint's GitHub Actions reporter benefits (inline annotations, structured error reporting)

## Possible Approaches

- Parse the GitHub reporter output format in addition to the default format
- Run oxlint with the GitHub reporter and extract results from exit code / structured output instead of parsing stdout
- Use oxlint's `--format=github` explicitly and emit our own summary alongside it

## References

- Fix PR: #914
- Original issue: https://github.com/voidzero-dev/setup-vp/issues/7