Exclude `vendor` from gbrain import - or add a .gbrainignore or similar to configure it on a directory bases

`brain import` imports php's `vendor` folder, this pollutes the database with tons of unrelated files.

It would be nice to either auto-exclude it - or better support something similar to .gitignore where we could ignore files and folders.

Start at the `brain import` entry point and trace how directories and files are selected for import. Compare the two requested outcomes—automatically excluding PHP `vendor` directories or supporting a `.gbrainignore`-style configuration—and confirm completion by showing that ignored or excluded files no longer pollute the database.

Written by the indexing model from the issue text.