feat(P0-001): scaffold monorepo structure

pnpm workspaces, Turborepo, TypeScript strict, ESLint flat config,
Prettier, Vitest, husky + lint-staged. Placeholder packages for all
16 packages + 2 apps + 2 plugins.

Closes #1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/gateway.Dockerfile

@@ -0,0 +1,22 @@
+FROM node:22-alpine AS base
+ENV PNPM_HOME="/pnpm"
+ENV PATH="$PNPM_HOME:$PATH"
+RUN corepack enable
+
+FROM base AS builder
+WORKDIR /app
+COPY pnpm-workspace.yaml pnpm-lock.yaml package.json ./
+COPY apps/gateway/package.json ./apps/gateway/
+COPY packages/ ./packages/
+RUN pnpm install --frozen-lockfile
+COPY . .
+RUN pnpm --filter @mosaic/gateway build
+
+FROM base AS runner
+WORKDIR /app
+ENV NODE_ENV=production
+COPY --from=builder /app/apps/gateway/dist ./dist
+COPY --from=builder /app/apps/gateway/package.json ./package.json
+COPY --from=builder /app/node_modules ./node_modules
+EXPOSE 4000
+CMD ["node", "dist/main.js"]