docs: add AGENTS.md with a prompt to get the viewer running

[bregydoc]

Summary

• Add AGENTS.md as the canonical entry point for coding agents asked to run or integrate Hyper: the uvx launch commands, accepted inputs, the Python API path, and the in-checkout commands.
• Link to it from a collapsed Ask a coding agent to run it block in the README, placed just before the Install uv instructions.

Why this shape

The first commit put an expanded ### For agents section above the human quick start. That introduced the only h3 in a README that is otherwise all h2 plus <details> blocks, and it restated the uvx command and wheel matrix that appear ~10 lines below — two places to update whenever either changes.

AGENTS.md is the conventional file agents look for and holds the detail once. The README block is collapsed like the existing Use pip instead of uv and Run from source blocks, so the one-line human command stays the first thing under the heading.

AGENTS.md also records the constraint agents most often get wrong: the viewer opens a native window on the machine running the command, so a remote host or container cannot display on the user's laptop.

Verification

• cargo run --locked -p hyper-viz --example project_scene — passes, no window
• cargo run --locked -- --help — exits cleanly with no window; confirms --help is headless-safe as documented. Args::parse() precedes all window setup in src/main.rs
• All five relative links in AGENTS.md resolve
• README heading structure re-checked: no stray h3, three consistent <details> blocks
• Not exercised: the published uvx path and the GUI itself