Singleton guard: show 'skipped (singleton)' as distinct status in execution history

[csoscd]

Summary

When a job is skipped because a previous instance is still running (singleton guard), no entry appears in the execution history. This makes it impossible to tell from the UI whether a job ran, failed, or was silently skipped.

Proposed Behaviour

A distinct "Skipped (singleton)" entry should appear in the execution history and timeline whenever a job is suppressed by the singleton guard, including a timestamp and — if available — the PID of the still-running instance.

Affected Component

• cron-wrapper.sh
• Agent
• Web UI

First Implementation Idea

1. New exit code constant — introduce exit code -6 ("Skipped – singleton") alongside the existing -5 ("Interrupted").
2. cron-wrapper.sh — when the singleton check detects a running instance, instead of silently exiting, call POST /execution/start immediately followed by POST /execution/finish with exit code -6 and a short output message (e.g. "Skipped: previous instance still running (PID 1234)").
3. Agent — ExecutionFinishEndpoint maps exit code -6 to status label "skipped_singleton"; no retry or notification is triggered for this status.
4. Web UI — history list and timeline render a distinct badge (e.g. grey "Skipped") for this status, visually separate from success, failure, and interrupted.