Add animated demo GIF to README

[rocketman-code]

Summary

• Replace the static code block hero section with an animated terminal recording (637KB GIF)
• Demo runs chainsaw on Cloudflare's workers-sdk monorepo, showing trace, --cut, and --chain on real data
• Move Install section directly below the GIF to reduce friction
• Include reproduction files: asciinema .cast, VHS .tape, and the shell script that drives the recording

Positioning rationale

The GIF sits in hero position (immediately after the tagline) following the pattern used by bat, fd, hyperfine, and delta. The static code example that was previously there is redundant with the GIF and has been removed -- detailed static examples remain in the Usage section below for reference and searchability.

Reproduction

Re-render the GIF at different sizes/themes:

# With agg (Rust, from asciinema)
agg docs/demo.cast docs/demo.gif --font-family "JetBrains Mono" --theme monokai

# With VHS (requires install)
vhs docs/demo.tape