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- session-start.sh: warns agents when CLI is outdated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/src/index.ts

@@ -13,6 +13,15 @@ import type { CommunicationStyle } from './types.js';
 
 export { VERSION, DEFAULT_MOSAIC_HOME };
 export { runWizard } from './wizard.js';
+export {
+  checkForUpdate,
+  backgroundUpdateCheck,
+  formatUpdateNotice,
+  getInstalledVersion,
+  getLatestVersion,
+  semverLt,
+} from './runtime/update-checker.js';
+export type { UpdateCheckResult } from './runtime/update-checker.js';
 export { ClackPrompter } from './prompter/clack-prompter.js';
 export { HeadlessPrompter } from './prompter/headless-prompter.js';
 export { createConfigService } from './config/config-service.js';