feature: Allow file completion outside a project

[necaris]

If not in a projectile or project.el project, use the agent-shell-cwd as the project root -- by default, though, list files only one level deep to minimize overwhelming responses.

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[xenodium]

Thanks for the PR.

I't be a shame to cap results when they could be genuinely useful for some directories that are not under revision control. Can't you disable agent-shell-completion-mode for the problematic directories?