test(importers): lock shared sink skip cache

[upsetbit]

Closes #77.

Summary

• add compile-time assertions that importertest.Sink implements importer.Sink and importer.SkipCache
• cover skip cache storage by session id and reason so importer tests keep exercising no_usage/state_seen idempotency

Local validation

• go test ./internal/importers/... -race
• devbox run -- just lint
• devbox run -- just test-race

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