docs(demo): re-record the demo with v0.4.0 features and stand-in project names #74

Part of #61. The last ticket of the map: it records the v0.4.0 features.

What

Rebuild demo/setup.sh and demo/demo.tape, re-record demo/demo.gif, and update the README demo section.

Projects shown

• No real project names: the current setup uses the maintainer's own projects (kryft.dev, wowaitech.com, grg, …). Remove them.
• Use names of well-known open-source repos instead, e.g. next.js, go, react, kubernetes, rust, deno, vscode, tailwindcss, django, linux. These are empty git init directories with one commit, not clones.
• Put them under ~/Code and ~/Work in the real home directory (not /tmp), so paths in the GIF read ~/Code/next.js and ~/Work/acme/api. Include a few nested ones and one non-git directory to show cdd add.
• setup.sh must refuse to run if ~/Code or ~/Work already exists. It must never delete anything it didn't create.
• cdd's own state must not touch the real config or History: point XDG_CONFIG_HOME, XDG_DATA_HOME and XDG_STATE_HOME at a demo directory (e.g. ~/.cache/cdd-demo).
• Add a teardown (demo/teardown.sh, or setup.sh --clean) that deletes ~/Code, ~/Work and the demo state directory, and run it after recording. Verify afterwards that they're gone.

Recording

• Slower: slower typing speed and longer pauses so a viewer can follow each step.

• Before each step, type a comment explaining it, then run the command. For example:

  # scan these dirs once to find every git repo under them
  cdd scan ~/Code ~/Work
  

• Show more of the features, in order:

  1. cdd scan
  2. Opening the Picker and filtering with a multi-word Query
  3. Enter to Jump, then pwd
  4. The key-hint footer
  5. A custom Action from a demo config, e.g. ctrl+l running a fake lazygit stand-in
  6. ctrl+y copy and ctrl+g remote, which leave the Picker open with a footer message (stub the browser opener so nothing really opens)
  7. cdd add on a non-git dir and seeing it in the Picker
  8. ctrl+d forget
  9. cdd config init

  Keep the whole GIF to a reasonable length (aim for under about 60s).

• Use VHS v0.11.0 (v0.12.0 silently writes no GIF). After recording, check that demo/demo.gif exists and actually changed, and look at a few frames.

Review gate

Do not merge. Open the PR and stop; the maintainer reviews the GIF before merging.

Acceptance

• No real project names appear anywhere in demo/, the tape or the GIF.
• ~/Code, ~/Work and the demo state directory are gone after teardown.
• The PR is open and unmerged, awaiting review.