[Stage 3] Draft agent evidence bundle schema

[Will-thom]

Goal: draft a provider-aware evidence bundle for coding agents without coupling the project to prompt formats.

Related plan: #82.

Depends on: #34, #90, and #79.

Acceptance criteria:
- Define additive JSON bundle fields for findings, commands, confidence reasons, relevant files, and provider context.
- Include report-only and PR-context examples.
- Avoid prompt-specific instructions or provider lock-in.
- Link to JSON compatibility expectations.

[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.