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>

.github/workflows/ci.yml

@@ -0,0 +1,64 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [22]
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+
+      - run: npm ci
+      - run: npm run typecheck
+      - run: npm test
+
+  docker:
+    runs-on: ubuntu-latest
+    needs: test
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Build Docker image
+        run: docker build -t cookiebridge:ci .
+
+      - name: Smoke test
+        run: |
+          docker run -d --name cb-test -p 8080:8080 cookiebridge:ci
+          sleep 3
+          curl -sf http://localhost:8080/health
+          docker stop cb-test
+
+  extension:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+          cache-dependency-path: extension/package-lock.json
+
+      - name: Install extension dependencies
+        working-directory: extension
+        run: npm ci
+
+      - name: Build all browsers
+        working-directory: extension
+        run: |
+          node esbuild.config.mjs --browser=chrome
+          node esbuild.config.mjs --browser=firefox
+          node esbuild.config.mjs --browser=edge
+          node esbuild.config.mjs --browser=safari