chore(scaffold): bootstrap docmost-sync Node/TS project skeleton

Set up the project structure per the new-project guide, adapted from the
Python skeleton to the Node/TS stack fixed in SPEC.md (reuses docmost-mcp).
Scaffold only — the sync engine is not implemented yet.

- src/settings.ts: single config layer on zod, schema keyed by real ENV
  names; credentials and own-service address have no default (fail fast).
- src/config-errors.ts: loadSettingsOrExit — clear startup message naming
  the missing/invalid env var instead of a raw stack trace; exit(1).
- src/index.ts: thin entry point that validates config and logs (stub).
- test/: vitest unit tests for settings parsing and config errors (10 tests).
- Makefile (install/env/build/test/run/dev/clean), strict tsconfig, vitest.
- Dockerfile (single-stage, no EXPOSE, prunes dev deps), docker-compose
  (daemon, volume on /app/data, watchtower), ghcr CI with build needs test.
- .env.example, .gitignore/.dockerignore, AGENTS.md, README.md.
- Pinned deps (dotenv, zod) + committed package-lock.json.

Dockerfile

@@ -0,0 +1,20 @@
+FROM node:22-slim
+
+WORKDIR /app
+
+# Dependencies first (better layer caching): copy manifests, install from lock.
+COPY package.json package-lock.json ./
+RUN npm ci
+
+# Runtime state directory (mounted as a volume in production).
+RUN mkdir -p data
+
+# Source + TS config, then compile to build/.
+COPY tsconfig.json ./
+COPY src/ src/
+RUN npm run build
+
+# Drop dev dependencies (typescript, tsx, vitest) to slim the runtime image.
+RUN npm prune --omit=dev
+
+CMD ["node", "build/index.js"]