Include dotfiles in @-file search suggestions

What feature would you like to see?

• Show files and directories that start with a dot (e.g., .github, .env, .vscode) in the suggestions when using the "@" file search.
• Keep honoring .gitignore (and related ignore files) exactly as today; do not surface files that Git ignores.
• It might be helpful to make this configurable (global and/or per-project), e.g., a toggle to include hidden files.

Are you interested in implementing this feature?

Yes. I can implement it.

Additional information

The @ search in the UI goes through codex-rs/file-search and uses ignore::WalkBuilder, which already honors .gitignore. Hidden files are excluded by default; this request is to optionally include them without changing .gitignore semantics.