Make ast-grep path configurable

[stephane-martin]

Feature request

I installed ast-grep inside my python/uv project using uv add ast-grep-cli.

So ast-grep is not really in my PATH, but it could be started with uv run ast-grep.

Currently ast-grep-mcp merely assumes the ast-grep command can be found in PATH, and there is no way to tell it to execute uv run ast-grep instead.