[quality] test: cover restart-storm drift rollup helpers (appendDriftSignal, applyAgentRestartResetBaselines edges, commonProblemReason)

[kubestellar-hive]

Test Improvement

Adds src/pkg/hub/drift_signal_append_test.go covering the read-time helpers that fold agent restart storms into a hive's DriftReport and the fleet rollup's common blocked-reason string:

• appendDriftSignal (pkg/hub/saas.go, was 0%, now 100%): nil-report guard, Count/Signals lockstep, worst-wins WorstSeverity escalation, and no downgrade when a milder signal follows a critical one.
• applyAgentRestartResetBaselines (pkg/hub/saas.go, was 75%, now 100%) — edge branches only; the happy path stays in agent_capability_test.go: nil resets no-op, per-agent isolation, unparseable ResetAt keeps attribution but not the delta, >24h reset expiry, negative-delta clamp to zero.
• commonProblemReason (pkg/hub/agent_capability.go, was 50%, now 100%): blank/whitespace next, first-wins, identical-keep, disagreement → "mixed".

Verified with go test -run 'TestAppendDriftSignal|TestApplyAgentRestartResetBaselines|TestCommonProblemReason' ./pkg/hub/ — all pass; go tool cover -func confirms all three functions at 100%.

Overlap check

Compared against every open quality/architect PR: #6091 is docs-only (fleet drift-signals operator reference, no test files); drift_test.go/drift_identity_test.go cover computeDrift in drift.go, not these saas.go/agent_capability.go helpers. No open PR touches these functions.

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Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-fable-5

[kubestellar-prow]

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[github-actions]

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