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

README.md

@@ -13,6 +13,10 @@ scripting**.
 > tier, so the first load after it's been idle takes ~30–60s to wake up.)
 >
 > Prefer a tour first? The **[project page](https://parththakkar106.github.io/AI-DnD/)** loads instantly.
+>
+> Want the internals? The **[design notes](https://parththakkar106.github.io/AI-DnD/guide.html)**
+> walk through the context budgeting, the world-state referee and the memory bank, and state the
+> reasoning behind each one ([Markdown version](docs/GUIDE.md)).
 
 Built with FastAPI + SQLAlchemy on the backend and React (Vite) on the frontend, running on
 SQLite locally and Postgres in the cloud. Works with **any OpenAI-compatible endpoint**: Ollama
@@ -199,6 +203,9 @@ is worth.
 - `plan/` — the phased implementation plan this was built from, kept as a build log. All twelve
   phases are complete; the later files (11, 12) double as design notes for the state-revert and
   world-state work.
+- [`docs/GUIDE.md`](docs/GUIDE.md) — design notes: how each subsystem works and why it was built
+  that way, with the measurements behind the decisions. Also rendered as a
+  [reading page](https://parththakkar106.github.io/AI-DnD/guide.html).
 - `backend/.env.example` — the few environment variables the backend reads.
 - [`docs/self-review.md`](docs/self-review.md) — a full-codebase self-review pass and what came
   out of it. All correctness findings are resolved.