feat(T-572): map SKIPPED in lib/job-status + alle terminal-checks

- lib/job-status.ts: SKIPPED ↔ 'skipped' mapping in beide richtingen
- components/shared/job-status.ts: label "Overgeslagen" + neutrale italic styling
- actions/admin/jobs.ts: cancel-guard erkent SKIPPED als eindstatus
- app/api/cron/cleanup-agent-artifacts: SKIPPED ook opruimen na 7d
- lib/insights/agent-throughput: SKIPPED telt mee als terminal

ACTIVE_JOB_STATUSES bewust ongewijzigd — SKIPPED is afgerond.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

actions/admin/jobs.ts

@@ -19,7 +19,7 @@ export async function cancelJobAction(jobId: string) {
   })
 
   if (!job) throw new Error('Job niet gevonden')
-  if (job.status === 'DONE' || job.status === 'FAILED' || job.status === 'CANCELLED') {
+  if (job.status === 'DONE' || job.status === 'FAILED' || job.status === 'CANCELLED' || job.status === 'SKIPPED') {
     throw new Error('Job is al in eindstatus')
   }