files-widget: add fuzzy filename search and Enter-to-jump

[tmustier]

Summary

Improve /readfiles search so it works as a real filename finder instead of a simple substring filter.

Current behavior

• Search mode opens with /
• Filename matching is still basic
• files-widget/TODO.md still has:
  • Fuzzy match file names
  • Highlight matches, Enter to jump

Proposed behavior

• Add fuzzy filename matching in the file browser
• Highlight the matched segments in results
• Let Enter jump/open the currently selected result directly
• Keep the fast keyboard-driven flow already present in /readfiles

Why

This is one of the highest-value remaining UX improvements for files-widget after the recent comment editor and Markdown viewer fixes.

It also aligns with Discord feedback requesting a more FZF-like search experience.

Notes

• See .research/files-widget-discord-feedback-2026-04-19.md
• This should stay compatible with the current file tree and changed-file navigation model
• If scope grows, split "local fuzzy filename search" from any later "global/outside-CWD search" work

Credit

Prompted in part by Pi Discord feedback from avg8888.