feat: add policy decision adapter boundary

packages/decision-pipeline/src/policy-adapter.md

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+# Policy Decision Adapter
+
+## Purpose
+
+The policy decision adapter creates the translation boundary between policy evaluation and the decision pipeline.
+
+## Responsibility
+
+The adapter converts policy output into a standardized Decision contract.
+
+```
+Policy Result
+      |
+      v
+Policy Decision Adapter
+      |
+      v
+Decision
+```
+
+## Non-Responsibilities
+
+The adapter does not:
+
+- Define policy rules
+- Evaluate domain events
+- Execute actions
+- Communicate with external systems
+
+## Design Goal
+
+Policies should remain independent from the pipeline while decisions remain consistent across future implementations.

packages/decision-pipeline/src/policy-adapter.ts

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+import type { Decision } from './decision';
+
+/**
+ * Boundary adapter between policy evaluation and decision output.
+ *
+ * This layer translates policy results into pipeline decisions.
+ * It does not own policy rules or execute actions.
+ */
+export interface PolicyDecisionAdapter {
+  adapt(policyResult: unknown): Promise<Decision>;
+}