Create Workspace

Create a new workspace OR (for Scott pushes) upsert one keyed on (captured_by_user_id, source_kind, source_id).

Two flows:

• No scott_push: classic workspace creation. Single transaction.
• With scott_push: idempotent. Same (user, source_kind, source_id) returns the same planspace. Push artifact metadata is replaced in place; scott_push turns are APPENDED past the existing chain tip (never deleted). Non-scott_push turns the user added in the Scott UI are never touched — they stay parented where the user put them and become branches at the prior tip after the next push.