files-widget: support browsing/searching outside the current working directory

[tmustier]

Summary

Support browsing and searching outside the current working directory in /readfiles.

Current behavior

• The browser root is always process.cwd()
• Tree building and scanning are rooted at the current project directory
• .research/files-widget-discord-feedback-2026-04-19.md notes this is currently unsupported by design

Proposed behavior

Add a deliberate way to move above the current project root or otherwise broaden the visible filesystem scope.

Possible directions:

• parent-directory navigation from the current root
• an explicit "change root" action
• opt-in broader search scope outside the current CWD

Why

Today /readfiles behaves like a project-scoped browser. That is often correct, but it blocks workflows where the user wants to:

• inspect nearby files outside the repo root
• jump across sibling folders
• search more broadly without leaving Pi

Notes

• Keep this separate from symlink traversal behavior; that is related but distinct work
• Existing symlink issue: Symlinks not being shown on MacOS #9
• This is more of a product/UX decision than a narrow bug fix, so the issue should capture expected constraints clearly before implementation

Credit

Prompted in part by Pi Discord feedback from avg8888.