Option to set "on completion: delete" as default

[GrayStrider]

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🔖 Feature description

Option to set "on completion: delete" as default in extension settings

✔️ Solution

Would save time for users that create mostly one-off tasks and do not wish to keep them after completion.

❓ Alternatives

Input "on completion: delete" manually every time

📝 Additional Context

Currently, "on completion: keep" is treated as default, same as not having that flag at all.