chore: Add a Java development skill for the monorepo (#12797)

Adding a skill from the existing [sdk-platform-java GEMINI.md
file](https://github.com/googleapis/google-cloud-java/blob/main/sdk-platform-java/GEMINI.md).
Try to break the Java development specific portions out so that it can
be used by the rest of the modules in the monorepo.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: lqiu96

.agents/skills/java-development/SKILL.md

@@ -0,0 +1,67 @@
+---
+name: java-development
+description: General guidance on Java development practices, building, testing, and style in the monorepo. Use this skill when working on Java code across the repository.
+---
+
+# Java Development Guide
+
+This skill provides general guidelines for Java development inside the monorepo. It covers building, formatting, testing, and style conventions to ensure consistency across modules.
+
+## Workflow
+
+### 1. Building the Project
+
+The repository uses Maven as its primary build system.
+
+*   **Build All Modules**: To build all modules from the root of the repository, run:
+    ```bash
+    mvn install -T 1C -P quick-build
+    ```
+    > [!TIP]
+    > Use `-T 1C` to build modules in parallel (one thread per CPU core) and `-P quick-build` to skip unnecessary plugins for faster builds.
+*   **Build a Specific Module**: You can also run Maven commands within a specific module directory (e.g., `java-bigquery`) to build only that module.
+
+### 2. Code Formatting
+
+Code formatting is enforced using the `fmt-maven-plugin`.
+
+*   **Check Formatting**: To check for formatting issues without modifying files, run:
+    ```bash
+    mvn fmt:check -T 1C
+    ```
+*   **Apply Formatting**: To automatically format the code according to the project style, run:
+    ```bash
+    mvn fmt:format -T 1C
+    ```
+    > [!TIP]
+    > To save time, run `mvn fmt:format` within the specific module directory you are working on, rather than at the root.
+    > [!NOTE]
+    > Always run `mvn fmt:format` before committing changes to avoid build failures due to formatting.
+
+### 3. Testing Strategy
+
+*   **Unit Tests**: Traditional unit tests should be added for individual classes and methods. Run them using:
+    ```bash
+    mvn test -T 1C
+    ```
+*   **Integration Tests**: Many modules have integration tests that run against live services or emulators. These may require specific profiles or environment variables. Refer to the specific module's README for details.
+
+### 4. Style Guide
+
+Follow these general rules to maintain code quality and consistency:
+
+1.  **Minimize Visibility**: Default to the most restrictive access level possible. Avoid using `public` unless the class or method is intended to be part of the public API.
+2.  Avoid Fully Qualified Names: Use imports to keep class names short and readable, rather than using fully qualified names in the code.
+3.  **Avoid Obsolete APIs**: Do not call methods marked with `@ObsoleteApi` or `@Deprecated` unless there are no viable alternatives.
+4.  **Clean Diffs**: Avoid unnecessary formatting changes or whitespace modifications to keep diffs clean and easy to review.
+
+### 5. Dependency Management
+
+*   **Version Bumps**: Try not to bump any external dependency versions unless there is a known security vulnerability (CVE) or a critical bug fix.
+*   **New Dependencies**: Avoid introducing new external dependencies. If a new dependency is required, provide a strong justification in the pull request.
+*   **Standard Library First**: Prefer to use features from the Java standard library, followed by existing dependencies in the project (preferably Google-managed dependencies).
+
+### 6. Contribution Guidelines
+
+*   **Commit Messages**: Follow the [Conventional Commits](https://www.conventionalcommits.org/) specification. Include the module as the scope (e.g., `feat(spanner): ...`, `fix(bigquery): ...`).
+*   **Pull Requests**: All code changes must be submitted via a pull request and require review. Ensure you pull the latest changes from `main` and resolve any conflicts before submitting.