Remove deleted cron job sessions

Deleting a cron job removed its cron definition but left the agent:<agent>:cron:<jobId> session record behind. Operators could still see deleted jobs in the Control UI session dropdown even though the job itself was gone.

Ran session cleanup after durable cron-definition removal through the canonical session lifecycle mutation, added guarded cleanup for exact active-run markers, and fenced same-ID replacement across sibling CronService instances sharing the same cron store.

• Removes ghost cron sessions from the session store and Control UI after isolated/current jobs are deleted.
• Preserves active runs, shared main sessions, unrelated jobs, and per-run history governed by cron retention.
• Prevents same-ID recreation races by making replacement wait for the previous job's asynchronous session cleanup to finish.

• Added a dedicated remove-session-cleanup suite covering isolated sessions, active markers, replacement fencing, sibling services, and session reaper behavior.
• Documented real CLI/gateway proof against isolated SQLite state showing the cron job and base session were both gone after removal.
• Merged with 7 focused cron/session test files and 138 passing tests documented on the PR.