Document the engine's design decisions

The README says what the project does; nothing said why any of it works the
way it does. This adds design notes written to be read end to end: each
section states the decision, the reasoning, and what it cost.

docs/GUIDE.md is the readable source. docs/guide.html is the same material as
a self-contained reading page for the project site — no webfonts, no scripts
beyond a progress rail, so it also works saved to disk and opened offline.

Covers the context budget allocator, the propose-and-referee world-state
engine, the measured length-hint result, the memory bank's settled-action and
cursor rules, the two coordinate systems behind the summarization bugs, the
retry variant machinery, the egress fix, and the demo-key pinning. Closes with
the measured numbers, the known limitations, and a pointer to the cleanup
backlog in self-review.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PoBAfwRzHozF2bhZumxjPk

docs/index.html

@@ -130,6 +130,7 @@
 
     <div class="cta">
       <a class="btn btn-primary" href="https://ai-dnd-1gmp.onrender.com">Launch the live demo →</a>
+      <a class="btn btn-ghost" href="guide.html">Read the design notes</a>
       <a class="btn btn-ghost" href="https://github.com/parththakkar106/AI-DnD">View the source</a>
     </div>
     <p class="wake">No sign-up, no API key. Hosted on a free tier that sleeps — the first load takes ~30–60s to wake.</p>