docs: 27B cron Kubernetes bias mitigation

Document the finding that 27B defaults to K8s CronJob format but
follows explicit 'NOT Kubernetes' constraints. Prompt pattern:
'standard cron YAML (NOT Kubernetes) for...'

From #576 benchmarks. Closes #649, #652

docs/big-brain-27b-cron-bias.md

@@ -0,0 +1,32 @@
+# Big Brain 27B — Cron Kubernetes Bias Mitigation
+
+## Finding (2026-04-14)
+
+27B defaults to generating Kubernetes CronJob format when asked for cron configuration.
+
+## Mitigation
+
+Add explicit constraint to prompt:
+
+```
+Write standard cron YAML (NOT Kubernetes) for fleet burn-down...
+```
+
+## Before/After
+
+| Prompt | Output |
+|--------|--------|
+| "Write cron YAML for..." | `apiVersion: batch/v1, kind: CronJob` |
+| "Write standard cron YAML (NOT Kubernetes) for..." | Standard cron format without k8s headers |
+
+## Implication
+
+The bias is default behavior, not a hard limitation. The model follows explicit constraints.
+
+## Prompt Pattern
+
+Always specify "standard cron YAML, not Kubernetes" when prompting 27B for infrastructure tasks.
+
+## Source
+
+Benchmark runs in #576. Closes #649, #652.