feat(FN-4621): complete Step 3 — add worktrunk resolver and validator

Fusion-Task-Id: FN-4621
Fusion-Task-Lineage: fc1b0b13-9052-4378-86e3-3634d6c9db4e

packages/core/src/worktrunk-settings.ts

@@ -0,0 +1,74 @@
+import type { WorktrunkOnFailure, WorktrunkSettings } from "./types.js";
+
+const WORKTRUNK_ON_FAILURE_VALUES: readonly WorktrunkOnFailure[] = [
+  "fail",
+  "fallback-native",
+] as const;
+
+export const DEFAULT_WORKTRUNK_SETTINGS: Required<Pick<WorktrunkSettings, "enabled" | "onFailure">> &
+  Pick<WorktrunkSettings, "binaryPath"> = {
+  enabled: false,
+  binaryPath: undefined,
+  onFailure: "fail",
+};
+
+/** Merge global ← project field-by-field so partial project overrides don't
+ *  wipe global values. A project value of `undefined` is treated as "inherit
+ *  from global". */
+export function resolveWorktrunkSettings(
+  globalValue: WorktrunkSettings | undefined,
+  projectValue: WorktrunkSettings | undefined,
+): WorktrunkSettings {
+  const enabled = projectValue?.enabled ?? globalValue?.enabled ?? DEFAULT_WORKTRUNK_SETTINGS.enabled;
+  const binaryPath =
+    projectValue?.binaryPath ?? globalValue?.binaryPath ?? DEFAULT_WORKTRUNK_SETTINGS.binaryPath;
+  const onFailure =
+    projectValue?.onFailure ?? globalValue?.onFailure ?? DEFAULT_WORKTRUNK_SETTINGS.onFailure;
+
+  return {
+    enabled,
+    ...(binaryPath !== undefined ? { binaryPath } : {}),
+    onFailure,
+  };
+}
+
+/** Strict validator used by GlobalSettingsStore writes and CLI parsing. */
+export function validateWorktrunkSettings(value: unknown): WorktrunkSettings {
+  if (value === undefined || value === null) {
+    return {};
+  }
+
+  if (typeof value !== "object" || Array.isArray(value)) {
+    throw new Error("worktrunk settings must be an object");
+  }
+
+  const input = value as Record<string, unknown>;
+  const validated: WorktrunkSettings = {};
+
+  if (input.enabled !== undefined) {
+    validated.enabled = Boolean(input.enabled);
+  }
+
+  if (input.binaryPath !== undefined) {
+    if (typeof input.binaryPath !== "string") {
+      throw new Error("worktrunk.binaryPath must be a string when set");
+    }
+    const trimmed = input.binaryPath.trim();
+    if (trimmed.length === 0) {
+      throw new Error("worktrunk.binaryPath cannot be empty");
+    }
+    validated.binaryPath = trimmed;
+  }
+
+  if (input.onFailure !== undefined) {
+    if (typeof input.onFailure !== "string") {
+      throw new Error("worktrunk.onFailure must be a string when set");
+    }
+    if (!(WORKTRUNK_ON_FAILURE_VALUES as readonly string[]).includes(input.onFailure)) {
+      throw new Error('worktrunk.onFailure must be one of: "fail", "fallback-native"');
+    }
+    validated.onFailure = input.onFailure as WorktrunkOnFailure;
+  }
+
+  return validated;
+}