* fix(sessions): reimport archived sessions in their workspace
Archived agent records were treated as active imports, hiding their provider sessions permanently. Workspace-originated imports also discarded their workspace identity and created a duplicate workspace.
* fix(sessions): validate archived session restores
Restore archived imports under their existing Paseo agent identity, reject stale workspace ownership, and gate workspace targeting when the host cannot honor it.
* fix(sessions): roll back failed archived imports
If provider resume or history hydration fails, close any partial runtime, re-archive the provider session, and restore the original stored agent record.
* fix(sessions): validate restored import ownership
* fix(sessions): serialize concurrent restores
* fix(sessions): harden archived import recovery
* fix(sessions): validate archived import placement
* refactor(sessions): centralize provider session imports
Keep workspace placement rollback and stored-agent activation behind their existing owners so Session remains the wire boundary.
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