Fix acquisition completion timestamp

[besendorf]

Summary

• Set the acquisition completion timestamp before writing acquisition.json in non-streaming acquisitions.
• Preserve an already recorded completion timestamp during cleanup so metadata remains consistent.
• Handle StoreInfo errors and add regression tests for non-zero completed metadata.

Root cause

acquisition.json was written before Complete() populated Completed, so non-streaming acquisitions stored Go's zero timestamp (0001-01-01T00:00:00Z).

Fixes #85.

Validation

• GOCACHE=/tmp/go-build GOMODCACHE=/tmp/go-mod go test ./...