build(site): containerize local production validation

[lefoulkrod]

What changed

• Adds a minimal root agent guide for Arbor and the Ivy/Lattice styling boundary.
• Adds a containerized production build that syncs generated data, checks the site, and builds it without modifying the checkout.
• Makes the data-sync script resolve the repository from its own location and documents the container command.

This keeps local production validation reproducible without adding a host-side toolchain.

How it was tested

• docker build --platform linux/amd64 --file container/Dockerfile .
• node --test tests/release-notes.test.mjs
• node scripts/release-notes.mjs validate-fragments
• git diff --check

Release note

None: Adds developer build isolation and agent guidance only; the published site is unchanged.