Adopt shared foundation (helpers + reusable CI + shared config)

[bntvllnt]

Reduce copy-paste against the shared packages:

• Migrate any hand-rolled retention sweep / test-register to @vllnt/convex-helpers
• Consume the org reusable CI workflow (ci.yml / publish.yml) instead of a local copy
• Extend the shared @vllnt/typescript + @vllnt/eslint-config presets; drop the copy-pasted prettier config
• Generate llms.txt via the shared generator instead of a per-repo script