docs: bring the book to parity with the skill; PR-gate the book build

The cross-harness skill shipped (PR #17/#19) with README/CLAUDE.md/AGENTS.md
coverage, but the mdBook — the public disclosure surface — had zero mention
of it. By this project's own standard that is a parity defect: the flagship
consumable feature was undocumented where it is disclosed.

- New chapter "Consume it: the peltier skill" (docs/src/skill.md): install
  paths and the harness matrix, the ZeroClaw script-blocking variant, the
  gate order verify mode enforces, the refusal doctrine, and exactly which
  of these claims CI asserts. Numbers carry their CI and workload.
- SUMMARY.md, architecture.md (layout now lists the skill + mirror),
  reproduce.md, introduction.md: cross-links to the chapter.
- docs.yml: pull requests now BUILD the book (never deploy) — a broken
  chapter or dangling include is a red PR check, not a post-merge surprise.
  Also closes an audit finding: the book rebuilds when results/** changes,
  since case-study chapters {{#include}} evidence files from there.

Book build verified locally with mdbook 0.4.x (skill.html renders).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/src/SUMMARY.md

@@ -23,6 +23,7 @@
 
 # Reference
 
+- [Consume it: the peltier skill](./skill.md)
 - [Reproduce it yourself](./reproduce.md)
 - [The trust boundary (infra)](./infra.md)
 - [Roadmap & scope](./roadmap.md)