docs(spec): deterministic helper spawn and session-end ordering rules

The launcher becomes the spawn mechanism: a shell script runs the roster check, assigns the id, and launches with the helper instructions in order, where a model-followed startup instruction can skip a step. The in-session roster check stays as the safety net for raw launches and still splits a live anchor into crashed versus concurrent.

Session-end ordering was unhandled: a helper outliving the primary stranded a dirty worktree, since the helper may not commit and the agent allowed to is gone. The git ban on helpers is concurrency-scoped, so it lifts when the helper finds itself alone at wrap-up and the last agent out closes the door with the full wrap-up. The mirror case pauses too: a primary wrapping with live helpers stops at the commit and asks whether to sweep the helper's in-flight work, wait, or leave closing to the helper.

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