Add project scaffolding

Makefile, Dockerfile, CI workflow, prettier config, manifests for
Chrome (MV3) and Firefox (MV2), source directory structure, and
minimal test suite. All checks pass.

Dockerfile

@@ -0,0 +1,15 @@
+# node:22-slim (22.x LTS), 2026-02-24
+FROM node@sha256:5373f1906319b3a1f291da5d102f4ce5c77ccbe29eb637f072b6c7b70443fc36
+
+RUN apt-get update && apt-get install -y --no-install-recommends make && rm -rf /var/lib/apt/lists/*
+RUN corepack enable && corepack prepare yarn@1.22.22 --activate
+
+WORKDIR /app
+
+COPY package.json yarn.lock ./
+RUN yarn install --frozen-lockfile
+
+COPY . .
+
+RUN make check
+RUN make build