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.

test/config-errors.test.ts

@@ -0,0 +1,56 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { z } from 'zod';
+import { loadSettingsOrExit } from '../src/config-errors.js';
+
+describe('loadSettingsOrExit', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('returns the factory value and does not exit on success', () => {
+    const exitSpy = vi
+      .spyOn(process, 'exit')
+      .mockImplementation((() => undefined) as never);
+
+    const result = loadSettingsOrExit(() => ({ ok: true }));
+
+    expect(result).toEqual({ ok: true });
+    expect(exitSpy).not.toHaveBeenCalled();
+  });
+
+  it('prints a named-variable message and exits(1) on a ZodError', () => {
+    // Mock process.exit to throw so control stops at the exit point, mirroring
+    // the real exit-the-process behaviour without killing the test runner.
+    const exitSpy = vi.spyOn(process, 'exit').mockImplementation(((
+      code?: number,
+    ) => {
+      throw new Error(`exit:${code}`);
+    }) as never);
+    const writeSpy = vi
+      .spyOn(process.stderr, 'write')
+      .mockImplementation(() => true);
+
+    expect(() =>
+      loadSettingsOrExit(() => z.object({ FOO: z.string() }).parse({})),
+    ).toThrow('exit:1');
+
+    expect(exitSpy).toHaveBeenCalledWith(1);
+    const written = writeSpy.mock.calls.map((c) => String(c[0])).join('');
+    expect(written).toContain('Missing required variable(s)');
+    expect(written).toContain('FOO');
+  });
+
+  it('propagates a non-ZodError without exiting', () => {
+    const exitSpy = vi
+      .spyOn(process, 'exit')
+      .mockImplementation((() => undefined) as never);
+    const boom = new Error('x');
+
+    expect(() =>
+      loadSettingsOrExit(() => {
+        throw boom;
+      }),
+    ).toThrow(boom);
+    expect(exitSpy).not.toHaveBeenCalled();
+  });
+});