Return to the inbox after completing a task opened from it

Follow-up to #3610524, which added "return to the list you came from" and wired it into the pending-actions surfaces rendered through the shared PendingActionLinksTrait (the process trace, the My Orchestra actions page, the instances-list Workflow column). The inbox builds its own action links rather than using that trait, so it was the one pending-actions surface the feature missed.

This is not a regression: the inbox never carried a return target, before or after that change. Completing a task opened from the inbox has always chained to the workflow next step; it only became noticeable now that every other list returns the operator to itself.

Problem

A task opened from the inbox carries no return target, so completing it chains to the workflow next step instead of returning to the inbox. For a review whose decision is an embedded form, and which is the last interactive step, the run advances and the dispatcher has nothing left to show, landing the operator on "This step is no longer available." instead of back on their inbox.

Proposed resolution

InboxController::inbox() threads its own URL as the orchestra_return target onto each completion-bound link (open, complete, and complete per outcome), through the existing OrchestraReturn::embed() helper. Every completion surface reached from the inbox already honors orchestra_return (the review comment form, the user and comment task forms, the interaction-task signal, the inbox complete route, and the collect round-trip remember and recall), so no other change is needed.

Remaining tasks

Kernel coverage asserting the inbox open and completion links carry the return.