Issue #3611213: Return to the inbox after completing a task opened from it

Follow-up to #3610524. That issue added return-to-list and wired it into the pending-actions surfaces rendered through PendingActionLinksTrait (the trace, the My Orchestra actions page, the instances-list column). The inbox builds its own action links, so it was the one pending-actions surface the feature missed.

Not a regression: the inbox never carried a return target before or after #3610524; a task completed from it always chained to the next step. For a review whose decision is an embedded form and which is the last interactive step, that lands the operator on "This step is no longer available." instead of the inbox.

Fix: InboxController::inbox() threads its own URL as the orchestra_return target onto each completion-bound link (open, complete, complete per outcome) via OrchestraReturn::embed(). Every completion surface reached from the inbox already honors orchestra_return, so no other change is needed.

Test: WorkItemPresentationTest::testInboxLinksCarryReturn asserts the inbox open and completion links carry the return (fails on a build without the threading).