Add --config flag and AST_GREP_CONFIG env var support

[sebthom]

This PR adds support for providing a custom sgconfig.yaml file to the mcp server, either via a --config CLI argument or via a AST_GREP_CONFIG. The env var is esp. helpful when checking .mcp.json files into git without needing to hardcode a config path.

Being able to specify a custom config file is useful when e.g. needing to registering custom languages with ast-grep.

The mcp server now also supports a --help flag which results in:

> python main.py --help
usage: python main.py [-h] [--config PATH]

ast-grep MCP Server - Provides structural code search capabilities via Model Context Protocol

options:
  -h, --help     show this help message and exit
  --config PATH  Path to sgconfig.yaml file for customizing ast-grep behavior (language mappings, rule directories, etc.)

environment variables:
  AST_GREP_CONFIG    Path to sgconfig.yaml file (overridden by --config flag)

For more information, see: https://github.com/ast-grep/ast-grep-mcp

[Copilot]

Pull Request Overview

This PR adds configuration file support to the ast-grep MCP server, allowing users to specify custom sgconfig.yaml files through a --config CLI flag or AST_GREP_CONFIG environment variable. This enhancement enables customization of ast-grep behavior such as registering custom languages and configuring rule directories.

• Added command-line argument parsing with --config flag and --help support
• Implemented environment variable support for AST_GREP_CONFIG with proper precedence handling
• Updated all ast-grep command invocations to include the config path when specified

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
main.py	Added argument parsing, config path resolution with validation, and updated all ast-grep subprocess calls to include config parameter
README.md	Added documentation section explaining custom configuration file usage

[sebthom]

@HerringtonDarkholme I addressed the CoPilot findings

[HerringtonDarkholme]

I don't know if adding command line argument or using an environment variable is the standard way to initialize MCP. But let's merge this first and see the feedback from the community.