This is based on a Elixir Phoenix application, but should work for elixir-only apps as well (they will have fewer directories, that's all).
A typical project would have these directories and files:
_build
assets
  css
  js
  node_modules
  package-lock.json
  package.json
  tailwind.config.js
  vendor
config
  config.exs
  dev.exs
  prod.exs
  runtime.exs
  test.exs
deps
lib
mix.exs
mix.lock
priv
test
For such a project I would probably add these to the ignore file:
_build
assets/node_modules
priv
# deps <- if the context is too large
With Gemini 2 Pro, and paid plan, this should be less of a problem