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/architecture.md

@@ -19,6 +19,9 @@ config/            accept.toml (thresholds), pricing.toml (ROI inputs)
 targets/           Vendored OSS targets — the only agent-writable path
 corpora/           Hash-pinned golden-replay inputs (read-only to agent)
 results/           Calibration evidence, case studies, generated reports
+.claude/skills/    The trust layer as a consumable Agent Skill — mirrored
+                   byte-identically at .agents/skills/ (CI-enforced); see
+                   "Consume it: the peltier skill"
 ```
 
 ## The seven-tool boundary