Curate markdown task `dependsOn`

[hittjwX]

• Find orphan markdown task IDs referenced in dependsOn value,
• Use git grep or similar to focus on the committed repo base,
• Default is to report on orphan or duplicate markdown task ID,
• User will manually resolve duplicate task IDs, then commit.
• Script will purge orphan task IDs in dependsOn to clean up.
• The objective is to have a clean dependsOn for task trees.

[hittjwX]

This would complement a script for purchasing aged, cancelled, or completed tasks.