[Stage 3] Define GitLab parity boundary for provider adapters

[Will-thom]

Goal: define what GitLab support must share with GitHub adapters and what can remain provider-specific.

Related plan: #82.

Depends on: #33, #84, and #85.

Acceptance criteria:
- Identify shared adapter concepts across GitHub and GitLab.
- Identify GitLab-specific MR and pipeline metadata differences.
- Keep GitLab support optional and investigation-first.
- Recommend whether implementation should wait for GitHub adapter stabilization.

[github-actions]

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

I appreciate the time you spent opening this issue. 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.

[Will-thom]

Maintainer update: moved to status: ready after the Stage 3 readiness gate (#79), v0.3.0 acceptance checklist (#100), and visual dependency map (#101). This is design/contract work; implementation-heavy provider issues remain blocked until related contracts land.