Replace default Hextra favicons and web manifest with agentregistry branding - #62

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: peaceiris/actions-hugo@75d2e84710de30f6ff7268e08f310b60ef14033f. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/