docs(readme): animated hero of agents in a terminal multiplexer

[pronskiy]

What & why

Adds an animated hero under the README intro: four agents write to one Omnigraph graph at once, each on its own branch in a tmux-style pane. Main takes three merges and refuses the fourth, a decision that contradicts one already merged, while the graph pane shows each write land. It shows branching, merging and conflict refusal in about 14 seconds.

Backing issue / RFC

• Trivial fast-lane (docs only), no issue/RFC required

Checklist

• Change is focused (one logical change)
• Tests added/updated for behavior changes (N/A, README only)
• Public docs updated if user-facing surface changed (this is the docs change)
• Release note in changelog.d/ or the skip-changelog label: skip-changelog
• Reviewed against docs/dev/invariants.md (N/A, no code touched)

Local verification

• Rendered on the fork: https://github.com/pronskiy/omnigraph/tree/readme-hero-tmux
• Every terminal in the hero is a real recording of the omnigraph CLI (v0.11.0) against a local server: stored mutations, branch merge. The refusal is the real ReplacedBy @card(0..1) cardinality conflict.

Notes for reviewers

• Adds two binaries: assets/readme/hero.webp (2.0 MB, looping) and assets/readme/hero.png (387 KB, shown with prefers-reduced-motion).
• Dark only, on its own background, so it looks the same in GitHub's light and dark themes.