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.

Makefile

@@ -0,0 +1,31 @@
+.DEFAULT_GOAL := help
+
+.PHONY: help install env build test run dev clean
+
+help: ## Show this help
+	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) \
+		| awk 'BEGIN {FS = ":.*?## "}; {printf "  \033[36m%-12s\033[0m %s\n", $$1, $$2}'
+
+node_modules: package.json package-lock.json
+	npm ci
+	@touch node_modules
+
+install: node_modules ## Install dependencies (npm ci)
+
+env: ## Create .env from the template if missing
+	@test -f .env || cp .env.example .env
+
+build: install ## Compile TypeScript to build/
+	npm run build
+
+test: install ## Run the test suite
+	npm test
+
+run: build ## Build and run the app
+	node build/index.js
+
+dev: install ## Run in watch mode (tsx)
+	npm run dev
+
+clean: ## Remove build artifacts and node_modules
+	rm -rf build node_modules