Refactor to Self-Hosted Dagger Platform & Upstream Updates

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This PR completes the refactor of the coding agent platform, replacing Vercel proprietary infrastructure with a self-hosted Dagger container system.

Key Changes:

Dagger Migration: Replaced Vercel sandboxing with an isolated Dagger container system (lib/dagger/).
README Rewrite: Updated README.md to reflect the de-Vercel'd, self-hosted nature of the project.
Security Updates: Applied upstream security patches, upgrading Next.js and React.
Model Support: Added support for new models, including Claude Sonnet 4.5 and Opus 4.5.
Universal Deployment: Added Dockerfile, nixpacks.toml, and updated docker-compose.yml for easy self-hosting (supports Docker, Dokploy, etc.).
Scaffolding Tool: Created create-web-cli-agents CLI package in packages/ to allow users to scaffold their own instances of the platform.