Improve ranger previews and codify required tooling

[m1ngsama]

Summary

The repo currently tracks ranger/scope.sh, but it does not capture the minimal rc.conf overrides or the package expectations needed to reproduce the richer ranger setup now running on 192.168.31.20.

Proposed changes

• add a minimal ranger/rc.conf with the preview and trash workflow overrides
• extend ranger/scope.sh to generate video and PDF thumbnails and use chafa as a readable fallback for images
• document the recommended Arch Linux packages for the preview pipeline
• mention ranger in the top-level README

Acceptance criteria

• the repo contains a reusable ranger config instead of machine-local drift
• image, video, and PDF previews are covered by the tracked config
• the package expectations are documented clearly enough to bootstrap a new machine