Hide worker log artifacts from assignments

[nathanwhit]

Summary

• keep recording remote stdout/stderr worker logs as artifacts, but exclude worker_log artifacts from manager-facing assignment rows
• exclude worker logs from manager artifact counts and the objective artifact strip
• add regression coverage so real artifacts still show while worker logs stay out of the assignment board

Tests

• go test ./internal/eventstore ./internal/orchestrator
• npm --prefix web test -- AssignmentBoard.test.tsx
• npm --prefix web run build