feat(FN-4400): complete Step 2 — add heartbeat prompt template settings

Fusion-Task-Id: FN-4400
Fusion-Task-Lineage: 0d4f9c48-5b8b-49eb-9cf3-d1d585ffe7fc

packages/core/src/settings-validation.ts

@@ -1,6 +1,7 @@
 import type {
   DirectMergeCommitStrategy,
   GithubAuthMode,
+  HeartbeatPromptTemplate,
   HeartbeatScopeDisciplineMode,
   UnavailableNodePolicy,
 } from "./types.js";
@@ -14,6 +15,10 @@ const HEARTBEAT_SCOPE_DISCIPLINE_MODES: readonly HeartbeatScopeDisciplineMode[]
   "lite",
   "off",
 ] as const;
+const HEARTBEAT_PROMPT_TEMPLATES: readonly HeartbeatPromptTemplate[] = [
+  "default",
+  "compact",
+] as const;
 
 /**
  * Validates a project unavailable-node routing policy value.
@@ -83,3 +88,16 @@ export function validateHeartbeatScopeDisciplineMode(value: unknown): HeartbeatS
     ? (value as HeartbeatScopeDisciplineMode)
     : undefined;
 }
+
+/** Returns a validated heartbeat prompt template for project/agent settings, otherwise undefined. */
+export function validateHeartbeatPromptTemplate(value: unknown): HeartbeatPromptTemplate | undefined {
+  if (value === undefined) {
+    return undefined;
+  }
+  if (typeof value !== "string") {
+    return undefined;
+  }
+  return (HEARTBEAT_PROMPT_TEMPLATES as readonly string[]).includes(value)
+    ? (value as HeartbeatPromptTemplate)
+    : undefined;
+}