ET-006 was handed to two different tasks because M-6 derives work_item_id from
the Plane sequence_id, which can collide -> the two tasks shared a branch/worktree
slug prefix and stepped on each other.
2a: ensure_unique_work_item_id() is a uniqueness-guard LAYERED ON TOP of the M-6
derive (derive is untouched): if the derived ET-NNN already exists in tasks for
the repo, it walks forward to the next free number. Applied in start_pipeline
after the derive.
2b (defense-in-depth): worktree is keyed by branch; if the resulting branch is
already owned by another task in the repo, disambiguate it with the unique
work_item_id + plane id so two tasks can never share a worktree.
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