feat(system-prompt): cache dynamic policy context

docs/core-data-structures/system-prompt.md

@@ -60,3 +60,23 @@ interface PromptSection {
   readonly text: string | ((context: AssembleContext) => string)
 }
 ```
+
+## Dynamic prompt context
+
+`PromptContext` is the cache-safe counterpart to `PromptSection`. The assembly resolves and orders these contributions, while agent-loop logs their complete current snapshot after retained model history only when it changed or compaction removed it.
+
+```ts type-equiv
+/**
+ * One dynamic model-context contribution. Unlike a {@link PromptSection}, its
+ * rendered text is materialized as a durable user-role snapshot at the request
+ * tail, so changing runtime state preserves the stable system/history prefix.
+ */
+interface PromptContext {
+  /** Unique name — a duplicate registration throws (see {@link SystemPrompt.context}). */
+  readonly name: string
+  /** Contexts are joined in ascending order, independently of system-section order. */
+  readonly order: number
+  /** Static text or a provider evaluated for each assembly. Empty text contributes nothing. */
+  readonly text: string | ((context: AssembleContext) => string)
+}
+```