docs: Add screen recordings for every CLI subcommand

[tony]

Summary

Adds a short terminal demo for every tmuxp subcommand and wires them into
the CLI reference, the homepage, and the README. Each demo shows the command's
real output — recorded against an isolated, PII-free sandbox — so readers see
what tmuxp ls, load, search, freeze, convert, shell, edit,
import, and debug-info actually do.

What's included

Four atomic commits:

• docs(demos) — asciinema casts + agg-rendered GIFs (cropped to content
  with a 10px sampled-background margin), VHS tapes with GIF/MP4/WebM renders,
  and setup-sandbox.sh, which builds the isolated demo sandbox.
• docs(sphinx) — the aspect_ratio extension (stamps each demo <img>
  with width:<W>px; max-width:100%; aspect-ratio:W/H so demos render at real
  size with no layout shift and narrow demos never upscale) and a demos/**
  build exclude (keeps the tape/cast/mp4/webm sources in the repo but out of
  the published site; referenced GIFs still reach the build via _images/).
• docs(cli) — the demo GIF on each of the nine subcommand pages, plus a
  load hero on the homepage (replacing the stale hand-recorded screencast).
• docs(README) — the load, ls, shell, freeze, convert, and
  debug-info demos under their sections.

Notes for reviewers

• Isolation. Demos record against a sandbox venv install of this checkout
  with HOME/XDG_CONFIG_HOME/TMUX_TMPDIR redirected, so every path on
  screen renders as ~/… or /tmp/tmuxp-demo/… and tmux runs on a throwaway
  server. debug-info is recorded with no server so its output stays compact
  and its own import path tilde-collapses. Re-render anything with
  bash docs/_static/demos/setup-sandbox.sh then the tape/cast.
• Real sizing over width:100%. The Sphinx hook indexes the embedded
  demos/asciinema GIFs last so a command's cropped size wins its basename
  over the same-named vhs/ source render; README <img> tags use real pixel
  widths so nothing is upscaled.
• README image URLs are absolute raw.githubusercontent.com/…/master/…
  links (required for PyPI's long-description rendering); they resolve once
  this merges to master.

Verification

uv run ruff format · uv run ruff check · uv run mypy · just build-docs
(clean, no new warnings) all pass. uv run pytest is green apart from a
pre-existing flaky spinner test (test_spinner_enabled_output) unrelated to
these docs-only changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.56%. Comparing base (0c9f8c2) to head (fa5fc9a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1077   +/-   ##
=======================================
  Coverage   82.56%   82.56%           
=======================================
  Files          31       31           
  Lines        2770     2770           
  Branches      518      518           
=======================================
  Hits         2287     2287           
  Misses        346      346           
  Partials      137      137           

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