fix: ensureWorkflowCrons creates missing crons instead of skipping all

[funsaized]

Summary

Fixes #322 — ensureWorkflowCrons now creates only missing agent crons instead of skipping all creation when any cron exists.

Problem

workflowCronsExist() used .some() to check if any cron existed for a workflow. If some crons were deleted during troubleshooting but others survived, the function returned true and ensureWorkflowCrons() skipped creating all crons. Missing agent crons were never recreated, causing workflow steps to get stuck at "pending" forever.

This was observed on a competitive-intel-scanner workflow (8 agents) where the landscaper cron was missing but product-scanner still existed.

Changes

• Replace workflowCronsExist() with getExistingCronAgentIds() that returns the set of agent IDs that already have crons
• ensureWorkflowCrons() now compares expected agent IDs (from the workflow spec) against existing cron agent IDs and creates only the missing ones
• setupAgentCrons() accepts an optional onlyAgentIds filter set so it can skip agents that already have crons while preserving the original index-based stagger timing

Testing

• Verified the project compiles cleanly with npm run build
• The fix is minimal and backward-compatible: when no crons exist, all are created as before; when all exist, none are created; when some are missing, only those are created

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