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/book.toml

@@ -0,0 +1,20 @@
+[book]
+title = "hotpath"
+description = "A profile-guided optimization agent that only ships wins it can prove."
+authors = ["hotpath"]
+language = "en"
+src = "src"
+
+[output.html]
+default-theme = "ayu"
+preferred-dark-theme = "ayu"
+git-repository-url = "https://github.com/bdbrown4/peltier.io"
+edit-url-template = "https://github.com/bdbrown4/peltier.io/edit/main/docs/{path}"
+additional-css = ["theme/custom.css"]
+
+[output.html.fold]
+enable = true
+level = 1
+
+[output.html.playground]
+runnable = false