docs: add AGENTS.md for apollo-java contribution workflow

[nobodyiam]

What's the purpose of this PR

Add a repository-local AGENTS.md for apollo-java so contribution workflow expectations are explicit and reusable for future AI-assisted and manual PRs.

Which issue(s) this PR fixes:

N/A

Brief changelog

• Add /AGENTS.md with:
  • module layout and common Maven commands
  • testing guidance for module/full test runs
  • commit/PR rules (single-commit PR expectation)
  • PR submission flow (push to personal fork, then open PR to apolloconfig/apollo-java)
  • CHANGES.md entry rule: description text should link to the PR URL

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

Notes:

• This is a docs-only change; no runtime code path is modified.
• CHANGES.md is not updated for this PR.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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