feat: add Vitest unit tests for utility functions

- Set up Vitest with 27 tests across 3 test suites
- relativeTime: edge cases, time buckets, future timestamps
- sessionDisplayName: labels, kinds, channels, UUID truncation
- messagesToMarkdown: roles, blocks, tool calls, system events
- Add test and test:watch npm scripts
- Add test step to CI workflow

.github/workflows/ci.yml

@@ -28,6 +28,9 @@ jobs:
       - name: Lint
         run: npm run lint
 
+      - name: Test
+        run: npm test
+
       - name: Type check
         run: npx tsc --noEmit