docs: embed the product demo video in the README (#126)

Adds a Demo section between the intro and Features, embedding a 78s product
demo video: an animated terminal typing real slackcli commands and painting
their real output across 13 scenes, from auth login-auto through search,
reading a thread from a permalink, sending, reacting, canvas-to-Markdown, and
--json piped into jq. No narration — legible on mute.

Hosted as a GitHub user-attachment rather than committed. The repo's .git is
~3.6 MB, so a 4.9 MB MP4 would have more than doubled clone size permanently,
and git keeps the blob even if the file is later deleted. Diff is +6 lines,
no binary.

Every command and line of output was verified against this repo's source
before rendering — commands from src/commands/*.ts, output shapes from
src/lib/formatter.ts, spinner lines from the real ora succeed strings — so
nothing on screen is a flag the CLI does not have.

The source that produces the video lives in
shaharia-lab/digital-marketing-assets under slackcli-demo/, so re-rendering
after a UI change adds no tooling here.

Closes #125