feat: implement M4 self-hosting, Docker & documentation

- Dockerfile with multi-stage build (Node 22 Alpine, sodium-native)
- docker-compose.yml with health check for easy self-hosting
- README with setup guide, API reference, and project overview
- Architecture docs (data flow, component breakdown, protocol constants)
- Security model docs (threat model, crypto primitives, self-hosting checklist)
- GitHub Actions CI pipeline (test, typecheck, Docker smoke test, extension builds)
- GitHub Actions release pipeline (GHCR push, extension zip artifacts)
- CONTRIBUTING.md with dev setup and code style guidelines

Co-Authored-By: Paperclip <noreply@paperclip.ing>

Dockerfile

@@ -0,0 +1,35 @@
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+
+# sodium-native needs build tools on Alpine
+RUN apk add --no-cache python3 make g++
+
+COPY package.json package-lock.json ./
+RUN npm ci --ignore-scripts=false
+
+COPY tsconfig.json ./
+COPY src/ ./src/
+RUN npm run build
+
+# --- Production image ---
+FROM node:22-alpine
+
+WORKDIR /app
+
+RUN apk add --no-cache python3 make g++
+
+COPY package.json package-lock.json ./
+RUN npm ci --omit=dev --ignore-scripts=false && \
+    apk del python3 make g++ && \
+    rm -rf /root/.npm /tmp/*
+
+COPY --from=builder /app/dist ./dist
+
+ENV PORT=8080
+ENV HOST=0.0.0.0
+EXPOSE 8080
+
+USER node
+
+CMD ["node", "dist/cli.js"]