Set completedAt when backlog tasks become done

Set completedAt when backlog tasks become done A successful transition to done leaves completedAt null.

Runnable reproduction / implementation sketch backlog_update({"taskId":"<disposable-task-id>","status":"pending"}) backlog_update({"taskId":"<disposable-task-id>","status":"done"}) backlog_get({"taskId":"<disposable-task-id>"}) Steps Create a disposable todo task. Move it to pending, then done. Fetch it and compare status, updatedAt, and completedAt. Delete the fixture. Safety Use a disposable task and delete it.

Observed status is done and updatedAt changes, but completedAt remains null.

Expected / proposed Transitioning to done should set completedAt; reopening should clear it.

Why it matters Completion latency and reporting cannot use the existing timestamp. Simple agents may treat null as incomplete state.