Ship arena as an npm package (`npx arena`)

[macanderson]

Context. Arena runs today via pnpm arena … (tsx) from a clone. To be adoptable by other teams it should be an installable package.

Scope

• Emit a runnable build (tsc → dist/, or a bundle) with an arena bin + shebang.
• Support npx arena@latest … and global install; ship tasks/ and pricing.json as package files.
• Publish workflow on tag (npm provenance / OIDC).

Acceptance. npx arena list works from an empty dir; versions are pinnable.