Persist per-session scope (identity, labels, attributes) so recovered crashes carry it #44

Recovered crashes currently carry no scope: a crashed session never persisted its own email/labels/attributes, and merging the new process's ambient scope into an old crash was misleading (and inconsistent between the launch-path and worker recovery passes), so #33 stopped merging it. Persist the scope into the session directory (allocation-free on the crash path: write on change, read at recovery) and have recovery prefer it.

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