Copy chapter slice into chapters/ and wire up includes

Part of #1006.

Copy `index.md`, `token-format.md`, `taxonomy.md`, `agent-surface.md` from `packages/design-data-spec/spec/` into the prototype's `chapters/` directory. Wire each up via `

Also add a `rules-excerpt.md` with 4–5 SPEC-NNN entries drawn from `rules/rules.yaml` to test stable-ID cross-references.

Start by reading the four source files in packages/design-data-spec/spec/ and the prototype's spec.bs. Copy index.md, token-format.md, taxonomy.md, and agent-surface.md into chapters/, wire each through the requested include syntax, and create rules-excerpt.md with 4–5 SPEC-NNN entries from rules/rules.yaml. Done means all chapters are included and stable-ID cross-references are exercised.

Written by the indexing model from the issue text.