Docs: add a TUI demo GIF to README (showcase interactive tool picker)

[primetimetank21]

Summary

Once the interactive TUI tool picker (#495) ships, add a short demo GIF to README.md showcasing it in action — arrow-key navigation, space to toggle tools, enter to confirm.

Why

A visual demo is the fastest way to communicate the interactive install UX to new / less-technical users (the primary audience the TUI is meant to help). Static flag docs undersell an interactive feature.

Scope

• Record the TUI with asciinema, render to a lightweight GIF (agg or gifski) — keep it small and crisp for GitHub rendering.
• Keep it short (~10–15s): show pre-checked defaults, arrow to move, space to toggle an opt-in tool (delta/lazygit), enter to confirm.
• Add it to the README near the "Selective Install" section.
• Caption must note the non-interactive path (--non-interactive / SETUP_NON_INTERACTIVE=1 / CI auto-detect) so the GIF does not imply the menu is mandatory for automation / devcontainers.
• Cross-platform: ideally show the bash menu; note the PowerShell equivalent.

Out of scope

• Full video / narrated walkthrough (a single looping GIF is enough).

Blocked by

• TUI tool picker: interactive keyboard-driven install menu (Phase 2 of #468) #495 (interactive TUI tool picker) — the feature must exist before it can be demoed.