v0.2.6 — remove broken demo.gif link (doc-only)

[0.2.6] - 2026-05-16

Fixed

• README Demo section no longer references a missing docs/demo.gif.
  Homebrew on macOS 15 (still pre-release) errors out installing vhs
  (Error: unknown or unsupported macOS version: :dunno), so the GIF
  can't be regenerated yet. The text snapshot now leads the section
  unconditionally, with a brief note about the future GIF.
• docs/demo.tape is unchanged — the moment vhs becomes installable
  on macOS 15 stable, vhs docs/demo.tape will regenerate the GIF and
  the README can re-embed it.

Doc-only patch.