Restore toast Undo on one-click completion now that row actions can declare a field write (objectstack#14092 closed) #146

Found by the 18.1.0 upgrade (PR #144, head fd0e541, report #139 comment 6412788813); not changed there because it changes behaviour.

Background

#4 / #41 shipped one-click completion with undo. The complete row action is a script action, and undoable on it was never fulfilled by any runtime; spec 18 removes the key, so #144 drops it. The one-click reversal stays the duly_task_undo row action (pinned by test/task-actions.test.ts), but the toast Undo the deck shows is gone.

objectstack#14092 (no declarative field write for a ROW action) is closed completed since 2026-09-03. If complete becomes an operation: 'update' row action, the platform's toast Undo may apply to it.

What done looks like

• Measure on 18.1.0: does a declarative row update action get a toast Undo, and does it still run the task hook that stamps completed_at / completed_late? Quote the platform source or docs.
• If yes: move complete (and skip if it fits) to the declarative form, keep the stamping and permission gates identical, keep duly_task_undo working, and test both.
• The headers of src/actions/task.handlers.ts and src/actions/task.actions.ts say no row-level declarative field write exists; correct them either way.

Restart-when: #144 is merged.

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