chore: add developer instructions (#32)

Author: averikitsch

DEVELOPER.md

@@ -0,0 +1,110 @@
+# DEVELOPER.md
+
+This document provides instructions for setting up your development environment
+and contributing to the Cloud SQL for MySQL Observability Gemini CLI Extension project.
+
+## Prerequisites
+
+Before you begin, ensure you have the following:
+
+1.  **Gemini CLI:** Install the Gemini CLI version v0.6.0 or above. Installation
+    instructions can be found on the official Gemini CLI documentation. You can
+    verify your version by running `gemini --version`.
+2.  **Cloud SQL for MySQL Instance:** For testing data plane tools, you will need access to an active Cloud SQL for MySQL instance.
+## Developing the Extension
+
+### Running from Local Source
+
+The core logic for this extension is handled by a pre-built `toolbox` binary. The development process involves installing the extension locally into the Gemini CLI to test changes.
+
+1.  **Clone the Repository:**
+
+    ```bash
+    git clone https://github.com/gemini-cli-extensions/cloud-sql-mysql-observability.git
+    cd cloud-sql-mysql-observability
+    ```
+
+2.  **Download the Toolbox Binary:** The required version of the `toolbox` binary
+    is specified in `toolbox_version.txt`. Download it for your platform.
+
+    ```bash
+    # Read the required version
+    VERSION=$(cat toolbox_version.txt)
+
+    # Example for macOS/amd64
+    curl -L -o toolbox https://storage.googleapis.com/genai-toolbox/v$VERSION/darwin/amd64/toolbox
+    chmod +x toolbox
+    ```
+    Adjust the URL for your operating system (`linux/amd64`, `darwin/arm64`, `windows/amd64`).
+
+3.  **Install the Extension Locally:** Use the Gemini CLI to install the
+    extension from your local directory.
+
+    ```bash
+    gemini extensions install .
+    gemini extensions link .
+    ```
+    The CLI will prompt you to confirm the installation. Accept it to proceed.
+
+4.  **Testing Changes:** After installation, start the Gemini CLI (`gemini`).
+    You can now interact with the `cloud-sql-mysql-observability` tools to manually test your changes
+    against your connected database.
+
+## Testing
+
+### Automated Presubmit Checks
+
+A GitHub Actions workflow (`.github/workflows/presubmit-tests.yml`) is triggered
+for every pull request. This workflow primarily verifies that the extension can
+be successfully installed by the Gemini CLI.
+
+Currently, there are no automated unit or integration test suites
+within this repository. All functional testing must be performed manually. All tools
+are currently tested in the [MCP Toolbox GitHub](https://github.com/googleapis/genai-toolbox).
+
+### Other GitHub Checks
+
+*   **License Header Check:** A workflow ensures all necessary files contain the
+    proper license header.
+*   **Conventional Commits:** This repository uses
+    [Release Please](https://github.com/googleapis/release-please) to manage
+    releases. Your commit messages must adhere to the
+    [Conventional Commits](https://www.conventionalcommits.org/) specification.
+*   **Dependency Updates:** [Renovate](https://github.com/apps/forking-renovate)
+    is configured to automatically create pull requests for dependency updates.
+
+## Building the Extension
+
+The "build" process for this extension involves packaging the extension's
+metadata files (`gemini-extension.json`, `cloud-sql-mysql-observability.md`, `LICENSE`) along with the
+pre-built `toolbox` binary into platform-specific archives (`.tar.gz` or `.zip`).
+
+This process is handled automatically by the
+[`package-and-upload-assets.yml`](.github/workflows/package-and-upload-assets.yml)
+GitHub Actions workflow when a new release is created. Manual building is not
+required.
+
+## Maintainer Information
+
+### Team
+
+The primary maintainers for this repository are defined in the
+[`.github/CODEOWNERS`](.github/CODEOWNERS) file:
+
+*   `@gemini-cli-extensions/senseai-eco`
+*   `@gemini-cli-extensions/cloud-sql-mysql-maintainers`
+
+### Releasing
+
+The release process is automated using `release-please`.
+
+1.  **Release PR:** When commits with conventional commit headers (e.g., `feat:`,
+    `fix:`) are merged into the `main` branch, `release-please` will
+    automatically create or update a "Release PR".
+2.  **Merge Release PR:** A maintainer approves and merges the Release PR. This
+    action triggers `release-please` to create a new GitHub tag and a
+    corresponding GitHub Release.
+3.  **Package and Upload:** The new release triggers the
+    `package-and-upload-assets.yml` workflow. This workflow builds the
+    platform-specific extension archives and uploads them as assets to the
+    GitHub Release.