Add output file E2E matrix

[Will-thom]

Summary

• add an end-to-end output file matrix using the compiled CLI binary
• cover text and JSON output with and without -output
• verify stdout behavior and use temporary files for generated reports
• document output-file E2E coverage in the testing guide

Validation

• go test ./cmd/prmaven -run TestCLIOutputFileMatrix -v
• .github/scripts/check-public-metadata.sh
• go test ./...
• git diff --check

Closes #74

[github-actions]

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