Document native integration scope

[Will-thom]

Summary

• Add an integrations guide that clarifies Stage 1 provider scope.
• Document that GitHub is the only first-party automation/example surface today.
• Clarify that the Stage 1 CLI has no native GitHub or GitLab API adapter and remains provider-agnostic.

Validation

• go test ./... with repository-local GOCACHE

[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

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