docs: add AGENTS.md for contribution workflow

Author: nobodyiam

AGENTS.md

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+# Repository Guidelines
+
+## Project Structure & Module Organization
+- Multi-module Maven repository with root `pom.xml`.
+- Core modules:
+  - `apollo-core`: shared constants/utilities.
+  - `apollo-client`: main Java client and Spring integration.
+  - `apollo-client-config-data`: Spring Boot ConfigData integration.
+  - `apollo-mockserver`: test/mock support.
+  - `apollo-openapi`: OpenAPI client.
+  - `apollo-plugin`: plugin modules.
+- Tests follow standard Maven layout: `*/src/test/java` and `*/src/test/resources`.
+- CI workflows are in `.github/workflows`.
+
+## Build, Test, and Development Commands
+- Build all modules: `mvn -B clean package -Dmaven.gitcommitid.skip=true`
+- Run all tests: `mvn clean test`
+- Run one module tests: `mvn -pl apollo-client clean test`
+- Run targeted tests: `mvn -pl apollo-client -Dtest=ClassNameTest test`
+- Compile only: `mvn -B clean compile -Dmaven.gitcommitid.skip=true`
+- Notes:
+  - This repository does **not** configure `spotless`; do not use `spotless:apply` here.
+  - Some integration tests log connection warnings in local/dev environments; focus on final Maven summary.
+
+## Coding Style & Naming Conventions
+- Java style: follow existing codebase conventions (Google-style Java patterns in current files).
+- Keep changes minimal and module-scoped.
+- Preserve existing package/class naming conventions.
+- Add/adjust tests for non-trivial behavior changes.
+
+## Testing Guidelines
+- JUnit 4 + JUnit Vintage are both used in this repo.
+- For bug fixes:
+  - Add a regression test first when feasible.
+  - Run module-level tests for changed modules.
+  - Prefer full `mvn clean test` before opening PR.
+
+## Commit & Pull Request Guidelines
+- Use Conventional Commits, e.g. `fix: ...`, `feat: ...`, `docs: ...`.
+- If applicable, include issue linkage in commit message body, e.g. `Fixes #88`.
+- Keep PR commits clean:
+  - Require a single commit in the PR branch before review (squash locally if needed).
+- Open PRs with `.github/PULL_REQUEST_TEMPLATE.md` and fill all sections with concrete content.
+- PR description should include:
+  - purpose/root cause
+  - change summary
+  - test commands actually run
+- PR submission flow:
+  - Push branch to personal fork remote first (for example `origin`).
+  - Open PR from `<your-fork>:<branch>` to `apolloconfig/apollo-java:main`.
+  - If history is rewritten for squash, update remote branch with `--force-with-lease`.
+
+## CHANGES.md Rules
+- Update `CHANGES.md` for user-visible fixes/features.
+- Use bullet style consistent with existing entries.
+- Entry format should be a Markdown link:
+  - link text = the actual change description
+  - link target = the PR URL (not issue URL)
+- Example:
+  - `[Fix ... detailed summary](https://github.com/apolloconfig/apollo-java/pull/123)`
+
+## Agent Workflow Hints
+- Reproduce first, then fix.
+- Prefer targeted module/test runs during iteration, then run broader tests before PR.
+- When creating upstream PRs:
+  - use a branch in personal fork
+  - base repository is `apolloconfig/apollo-java`
+  - ensure PR branch is squashed to one commit