docs: full mdBook site + GitHub Pages deploy; rewrite README as project front page

- docs/: an mdBook documentation site — thesis, architecture, measurement
  discipline, equivalence gates, agent loop, isolation/anti-reward-hacking,
  playbook, ROI, reproduce, roadmap, and the five case studies (pulled from
  their single source in results/ via {{#include}}). Builds clean (21
  pages).
- .github/workflows/docs.yml: builds mdBook and deploys to GitHub Pages on
  push to main (one-time setup: Settings → Pages → Source = GitHub Actions).
- README.md: rewritten as the project front page — the thesis, the verified
  results table, layout, quick start, how it stays honest, links to the
  book. Replaces the stale Phase-0-era text.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

docs/theme/custom.css

@@ -0,0 +1,20 @@
+/* hotpath docs — small refinements over the default mdBook theme. */
+
+.content h1 {
+    border-bottom: 2px solid var(--links);
+    padding-bottom: 0.2em;
+}
+
+/* Verified-number tables read as evidence: tighten and emphasize. */
+.content table td:first-child {
+    font-weight: 500;
+}
+
+blockquote {
+    border-inline-start: 4px solid var(--links);
+}
+
+/* Inline verdict-ish code (accepted / rejected-bench / needs-human-review) */
+.content code {
+    font-variant-ligatures: none;
+}