feat: install.sh + auto-update checker for CLI

- tools/install.sh: standalone installer/upgrader, curl-pipe safe
  (main() wrapper, process.argv instead of stdin, mkdir -p prefix)
- packages/mosaic/src/runtime/update-checker.ts: version check module
  with 1h cache at ~/.cache/mosaic/update-check.json
- CLI startup: non-blocking background update check on every invocation
- 'mosaic update' command: explicit check + install (--check for CI)
- session-start.sh: warns agents when CLI is outdated
- Proper semver comparison including pre-release precedence
- eslint: allow __tests__ in packages/mosaic for projectService

packages/mosaic/__tests__/update-checker.test.ts

@@ -0,0 +1,52 @@
+import { describe, expect, it } from 'vitest';
+import { semverLt, formatUpdateNotice } from '../src/runtime/update-checker.js';
+import type { UpdateCheckResult } from '../src/runtime/update-checker.js';
+
+describe('semverLt', () => {
+  it('returns true when a < b', () => {
+    expect(semverLt('0.0.1', '0.0.2')).toBe(true);
+    expect(semverLt('0.1.0', '0.2.0')).toBe(true);
+    expect(semverLt('1.0.0', '2.0.0')).toBe(true);
+    expect(semverLt('0.0.1-alpha.1', '0.0.1-alpha.2')).toBe(true);
+    expect(semverLt('0.0.1-alpha.1', '0.0.1')).toBe(true);
+  });
+
+  it('returns false when a >= b', () => {
+    expect(semverLt('0.0.2', '0.0.1')).toBe(false);
+    expect(semverLt('1.0.0', '1.0.0')).toBe(false);
+    expect(semverLt('2.0.0', '1.0.0')).toBe(false);
+  });
+
+  it('returns false for empty strings', () => {
+    expect(semverLt('', '1.0.0')).toBe(false);
+    expect(semverLt('1.0.0', '')).toBe(false);
+    expect(semverLt('', '')).toBe(false);
+  });
+});
+
+describe('formatUpdateNotice', () => {
+  it('returns empty string when up to date', () => {
+    const result: UpdateCheckResult = {
+      current: '1.0.0',
+      latest: '1.0.0',
+      updateAvailable: false,
+      checkedAt: new Date().toISOString(),
+      registry: 'https://example.com',
+    };
+    expect(formatUpdateNotice(result)).toBe('');
+  });
+
+  it('returns a notice when update is available', () => {
+    const result: UpdateCheckResult = {
+      current: '0.0.1',
+      latest: '0.1.0',
+      updateAvailable: true,
+      checkedAt: new Date().toISOString(),
+      registry: 'https://example.com',
+    };
+    const notice = formatUpdateNotice(result);
+    expect(notice).toContain('0.0.1');
+    expect(notice).toContain('0.1.0');
+    expect(notice).toContain('Update available');
+  });
+});