infra: restore build generators

[marc0olo]

Context

Build generator scripts from restructuring-attempt-1 produce derived artifacts. They need to be restored for the current structure.

Scripts to restore

• scripts/generate-llms-txt.mjs — generates /llms.txt discovery index (note: the agentDocs() Astro plugin in plugins/astro-agent-docs.mjs already handles this at build time — verify if this script is still needed or superseded)
• scripts/generate-manifest.mjs — generates a docs manifest for external tooling

Steps

1. Check plugins/astro-agent-docs.mjs — determine whether generate-llms-txt.mjs is redundant
2. Restore any scripts not covered by the Astro plugin, adapted for current page paths
3. Add to package.json as npm run generate