fix(alerts): encode sustained PromQL holds

[rldyourmnd]

Summary

• treat OpenObserve v0.92 trigger threshold as the documented PromQL series-coverage gate
• encode reviewed hold windows with min_over_time for high conditions and max_over_time for low conditions
• keep immediate rules immediate and preserve evaluation cadence

Verification

• go test -race ./internal/observabilityrules ./cmd/gha-fleet
• go vet ./internal/observabilityrules ./cmd/gha-fleet
• live v0.92 executed the generated subquery successfully
• git diff --check