Add CLI output file option

[Will-thom]

Closes #15.

Summary

• adds a global -output option for analysis commands
• writes text or JSON reports to the requested file path while keeping stdout behavior unchanged when absent
• documents output-file usage in README, usage docs, and JSON contract examples

Tests

• go fmt ./cmd/prmaven
• go test ./...
• git diff --check

[github-actions]

Hi @Will-thom, thank you for contributing to PR Maven CLI.

I appreciate the time you spent opening this pull request. This project is intentionally shaped for focused, reviewable open source contributions, so your input helps the tool become more useful for Java and Maven teams.

A maintainer will review the context, labels, and next steps as soon as possible. If this is your first contribution here, welcome aboard.

Useful links:

• Contributing guide: CONTRIBUTING.md
• Testing guide: docs/testing.md
• CI/CD guide: docs/ci.md

Thank you for helping build a deterministic, local-first Maven failure triage tool.