docs(demo): re-record the demo with v0.4.0 features and stand-in projects

[hammadmajid]

Refs #74. Not merging: waiting for maintainer review of the GIF.

What changed

• demo/setup.sh creates stand-in projects in the real home: ~/Code/{next.js,go,react,rust,oss/kubernetes,oss/deno} and ~/Work/{acme/api,acme/web,globex/billing}. They are empty git init repos with one dated commit and an origin remote, never clones. ~/Work/notes is a plain directory for cdd add.
  • cdd's config, History and projects file live in ~/.cache/cdd-demo (via XDG_*_HOME), so the real ones are never touched.
  • The script refuses to run if ~/Code, ~/Work or ~/.cache/cdd-demo already exists.
• New demo/teardown.sh deletes exactly those three directories. It does nothing unless setup's marker file is present.
• Stubs for xdg-open, open, lazygit and the clipboard tools mean nothing opens for real.
• The tape is slower (90ms typing, longer pauses), and each step is introduced by a # comment. Steps:
  1. cdd scan ~/Code ~/Work
  2. open the Picker and show the key-hint footer
  3. filter with a multi-word Query (oss kube), Enter, pwd
  4. a custom ctrl+l lazygit Action
  5. ctrl+y copy, then ctrl+g remote (stubbed opener, log shown)
  6. cdd add ~/Work/notes, then find it in the Picker
  7. ctrl+d then y to forget rust
  8. cdd config init, then head -20 of the result

Verification

• Recorded with VHS v0.11.0. The GIF is 1.8 MB and ~100 s long.
• Reviewed frames at 1 fps. Every step is visible, no real project names appear, and the prompt is neutral.
• Teardown ran, and ~/Code, ~/Work and ~/.cache/cdd-demo are gone.

Known in the recording

• The preview pane and the "copied" message show absolute paths under /home/<user>.
• The added non-git directory shows a red ! git could not read it status. That is a product bug, tracked separately.