Feature (all agents): Add selection field for popular MCP servers with auto-configuration

## Overview

When users create a template with any agent (Claude Code, Gemini CLI, etc.), an MCP server selection field should appear (similar to JetBrains IDE integrations). This field would list popular MCP servers like microsoft/playwright-mcp and GitHub MCP Server, allowing users to select servers based on their use case.

This feature aligns with Coder's goal of reducing project setup time by eliminating the need for users to manually configure each MCP server.

## Proposed Implementation

* Add an mcp-servers/ folder within each agent's template directory
* Store individual .sh setup scripts in this folder (e.g., playwright-mcp.sh, github-mcp.sh)
* In main.tf, reference the selected MCP server scripts and inject them into the coder_script or coder_utils post_install_script resource's startup script
* Execute the configuration automatically during workspace provisioning