feat(#13): add vitest infrastructure and unit tests for sidebar org/env builders#

Problem#

No test infrastructure existed for the frontend. The buildOrgViewItems and buildEnvViewItems functions (whose bug fix was already merged) had no unit tests covering their multi-issue scanning, deduplication, and merging behaviour.

Task / Link#

GitHub Issue #13

Changes#

• Added vitest as a devDependency and updated the test script from a no-op echo to vitest run
• Created vitest.config.ts with @/ path alias resolution pointing to src/
• Added src/lib/utils/__tests__/sidebar.test.ts with 13 unit tests covering:
  • buildOrgViewItems: empty root, missing directories, org deduplication, repo deduplication, run merging
  • buildEnvViewItems: empty root, missing tasks, env grouping, env deduplication, child merging

Notes#

• findNodeAtPath is unexported and tested indirectly through the public builder functions
• The CJS deprecation warning from Vite is cosmetic and does not affect test execution

Testing#

• npm run test — 13/13 tests pass
• npm run lint — zero warnings
• npm run build — succeeds