fix(onboarding): a partial import-scan failure can no longer read as Done

When some (not all) agents' dry-run scans failed, their contribution was silently dropped: the History rung could claim 'N sessions imported' or 'no prior history found' while a failed agent's real status was unknown — and since each agent is the sole source for its own history, nothing else could correct it. Failed scans are now recorded (ScanFailed) and the rung degrades honestly: pending imports from healthy agents stay actionable (Missing), but a rung that would otherwise read Done or n/a renders Unknown naming the failed agent. Never memoized, as before.

Trail-Finding: 019f7080-d1e9-713e-9317-b0070166bf10