Verify and publish to Packagist

[nficano]

composer.json declares arcp/arcp v0.1.0 but Packagist registration is unverified. Steps:

1. Submit package at https://packagist.org/packages/submit (if not done)
2. Set up GitHub webhook for automatic updates on push
3. Tag v1.1.0 when features complete
4. Verify composer require arcp/arcp succeeds

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)