Fix current errcheck lint failures

[marcus]

Summary

• fix the currently reproducible errcheck failures in command code by handling best-effort config/database writes explicitly
• clean up remaining errcheck findings surfaced during the same lint pass across adjacent tests and sync helpers
• add shared test helpers for common must-succeed setup operations to keep the fixes narrow and readable

Verification

• golangci-lint run ./...
• go test ./cmd -run 'Start|Unstart|Ws'
• go test ./... (still fails on the pre-existing unrelated internal/sync test TestColumnNameInjection_DroppedSilently)

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Automated by nightshift