chore: add initial docs

Author: Yuan325

POSTGRESQL.md

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+# PostgreSQL Extension
+
+This document provides instructions for the Gemini agent to assist users with the PostgreSQL extension.
+
+## PostgreSQL MCP Server (Data Plane: Connecting and Querying)
+
+This section covers connecting to a PostgreSQL database instance.
+
+1.  **Verify Environment Variables**: Before attempting to connect, confirm with the user that the following environment variables are set in the extension configuration or their shell environment.
+
+    *   `POSTGRES_HOST`: The hostname or IP address of the PostgreSQL server.
+    *   `POSTGRES_PORT`: The port number of the PostgreSQL server.
+    *   `POSTGRES_DATABASE`: The name of the database to connect to.
+    *   `POSTGRES_USER`: The username for authentication.
+    *   `POSTGRES_PASSWORD`: The password for authentication.
+
+2.  **Handle Missing Variables**: If a command fails with an error message containing a placeholder like `${POSTGRES_HOST}`, it signifies a missing environment variable. Inform the user which variable is missing and instruct them to set it.
+
+3.  **Handle Permission Errors**: If you encounter permission errors, ensure the user has the correct privileges on the PostgreSQL database.

README.md

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+# Gemini CLI Extension - PostgreSQL
+
+This Gemini CLI extension provides a set of tools to interact with [PostgreSQL](https://www.postgresql.org/docs/) instances. It allows you to manage your databases, execute queries, and explore schemas directly from the [Gemini CLI](https://google-gemini.github.io/gemini-cli/), using natural language prompts.
+
+## Features
+
+*   **Integrated with Gemini CLI:** As a Google-developed extension, it integrates seamlessly into the Gemini CLI environment, making security an accessible part of your workflow.
+*   **Connect to PostgreSQL:** Securely connect to your PostgreSQL instances.
+*   **Explore Database Schema:** List databases, tables, views, and schemas.
+*   **Query your Database:** Execute SQL queries against your database.
+
+## Supported Tools
+
+* list-tables: Use this tool to list tables and descriptions.
+* execute-sql: Use this tool to execute any SQL statement.
+
+## Prerequisites
+
+Before you begin, ensure you have the following:
+
+*   [Gemini CLI](https://github.com/google-gemini/gemini-cli) installed.
+*   A running PostgreSQL instance.
+
+## Installation
+
+To install the extension, use the `gemini extensions install` command:
+
+```bash
+gemini extensions install github.com/gemini-cli-extensions/postgresql.git
+```
+
+## Configuration
+
+*   `POSTGRES_HOST`: The hostname or IP address of the PostgreSQL server.
+*   `POSTGRES_PORT`: The port number of the PostgreSQL server.
+*   `POSTGRES_DATABASE`: The name of the database to connect to.
+*   `POSTGRES_USER`: The username for authentication.
+*   `POSTGRES_PASSWORD`: The password for authentication.
+
+
+## Usage
+
+* Explore Schemas and Data
+* Generate code
+
+
+## Security
+
+This extension executes commands against your PostgreSQL database. Always review the generated SQL queries before execution, especially for write operations.
+
+## Disclaimer
+
+This is not an officially supported Google product. This extension is under active development, and breaking changes may be introduced.

gemini-extension.json

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+{
+  "name": "PostgreSQL",
+  "version": "0.1.0",
+  "description": "Connect to PostgreSQL",
+  "mcpServers": {
+    "PostgreSQL": {
+      "command": "${extensionPath}${/}toolbox",
+      "args": [
+        "--prebuilt",
+        "postgres",
+        "--stdio"
+      ],
+      "env": {
+        "POSTGRES_HOST": "${POSTGRES_HOST}",
+        "POSTGRES_PORT": "${POSTGRES_PORT}",
+        "POSTGRES_DATABASE": "${POSTGRES_DATABASE}",
+        "POSTGRES_USER": "${POSTGRES_USER}",
+        "POSTGRES_PASSWORD": "${POSTGRES_PASSWORD}"
+      }
+    }
+  },
+  "contextFileName": "POSTGRESQL.md"
+}