Split the AsDecided product site and knowledge base

[tcballard]

Summary

• make the ChatGPT Sites experience the concise, high-end AsDecided product
  front door
• make GitHub Pages the searchable knowledge base for the specification,
  concepts, practical guides, and reference
• source authoritative documentation at build time from asdecided/core and
  asdecided/spec, rather than maintaining another copy
• adopt the approved stepped decision-link mark, black/yellow palette, and
  Build, as decided. identity across both surfaces
• add a strict knowledge-base build and GitHub Pages deployment workflow

Architecture

This repository now owns two deliberately different public surfaces:

1. Product site — ChatGPT Sites. A short, authored introduction to the
   product, its trust boundary, native installation, and ecosystem.
2. Knowledge base — GitHub Pages. A navigable documentation system for
   learning the model, implementing the specification, operating Core, and
   finding exact reference material.

The knowledge-base shell and navigation live here. Normative RAC material
remains owned by asdecided/spec; product behavior and command documentation
remain owned by asdecided/core. The deployment vendors those sources
deterministically during the build.

Review and cutover

The current public asdecided.github.io deployment remains unchanged while
this PR is open. The product experience is available as an owner-only ChatGPT
Sites preview. Merging this PR deploys the knowledge base to GitHub Pages; the
ChatGPT Sites deployment can then be made public and connected to the chosen
product domain separately.

Validation

• npm run build
• node --test tests/rendered-html.test.mjs
• ./scripts/vendor-knowledge.sh
• strict MkDocs build with pinned dependencies
• in-app browser review on desktop and at 390×844
• no horizontal overflow at 390px
• responsive knowledge-base navigation verified
• product-site interactive query tabs and scroll reveal verified
• no browser warnings or errors