Improve CLI help output

[Will-thom]

Summary

• add dependency-free built-in help for -h, --help, -help, and help
• include commands, flags, examples, and exit codes in the help output
• document the help entrypoints in README and usage docs

Validation

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

Closes #19

[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.