Centralize viewer URL construction and E2E_REPOS#

Problem#

E2E_REPOS, VIEWER_BASE, and the viewerUrl() function were duplicated in three page components, making them a silent drift risk with agent-core's canonical E2E_SOURCE_REPOS list.

Task / Link#

Closes #41

Changes#

• Added lib/viewer.ts with shared E2E_REPOS, VIEWER_BASE, getArtefactsSubdir(), and getViewerUrl() helpers; doc comment references agent-core as source of truth
• Added lib/viewer.test.ts with 6 unit tests covering e2e/production subdir selection and full URL shape
• Added vitest as dev dependency with a test script
• Refactored app/pipeline/page.tsx, app/pipeline/[taskId]/page.tsx, and app/agents/page.tsx to import getViewerUrl from lib/viewer (removed all local duplicates)

Notes#

E2E_REPOS is still a manual mirror of agent-core's E2E_SOURCE_REPOS. The source-of-truth comment and unit tests make drift detectable at review/CI time.

Testing#

• npm test — 6/6 vitest unit tests pass
• npm run lint — no ESLint warnings or errors
• npm run build — compiles successfully