docs(demo): add a docs/demo/RECORDING.md asciicast capture guide

[omerakben]

Context. The two demo scripts (`bun run demo:todo-cli` and `bun run demo:failure-gates`) produce clean, readable terminal output that would be great content for tweets and HN posts. Right now there's no recipe for capturing them as asciinema/asciicast files, so community contributors can't share recordings.

Ask. Add a short docs/demo/RECORDING.md that:

1. Lists the tools needed (asciinema, optionally agg for GIF conversion)
2. Shows the exact command to capture the failure-gates demo
3. Shows the exact command to capture the todo-cli demo
4. Suggests where to host the resulting .cast files

Why this is a good first issue. Pure docs work; no code changes; lets future contributors share their own recordings.

Acceptance.

• docs/demo/RECORDING.md exists
• Both demo capture commands are documented
• README links to it from the demo section