fix(walkthrough): use remote default branch

packages/ui/src/components/views/git/baseBranch.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, test } from 'bun:test';
+import { deriveBaseBranch, hasResolvableBaseBranch } from './baseBranch';
+
+describe('deriveBaseBranch', () => {
+  test('prefers the remote default branch hint over conventional fallbacks', () => {
+    expect(deriveBaseBranch({
+      remoteNames: new Set(['origin']),
+      localBranches: ['next'],
+      rootBranchHint: 'origin/react',
+    })).toBe('react');
+  });
+});
+
+describe('hasResolvableBaseBranch', () => {
+  test('rejects the main fallback when it does not exist', () => {
+    expect(hasResolvableBaseBranch({
+      baseBranch: 'main',
+      localBranches: ['next', 'react'],
+      remoteBranches: ['origin/next', 'origin/react'],
+    })).toBe(false);
+  });
+
+  test('accepts a base branch available through a remote-tracking ref', () => {
+    expect(hasResolvableBaseBranch({
+      baseBranch: 'main',
+      localBranches: ['next'],
+      remoteBranches: ['origin/main', 'origin/next'],
+    })).toBe(true);
+  });
+});

packages/ui/src/components/views/git/baseBranch.ts

@@ -62,3 +62,18 @@ export const deriveBaseBranch = (options: {
   if (localBranches.includes('develop')) return 'develop';
   return 'main';
 };
+
+/**
+ * Whether a base branch can be resolved locally or through one of the active
+ * remote-tracking refs. Callers must not offer comparisons against the `main`
+ * fallback when that ref does not actually exist in the repository.
+ */
+export const hasResolvableBaseBranch = (options: {
+  baseBranch: string;
+  localBranches: readonly string[];
+  remoteBranches: readonly string[];
+}): boolean => {
+  const { baseBranch, localBranches, remoteBranches } = options;
+  return localBranches.includes(baseBranch)
+    || remoteBranches.some((branch) => branch.endsWith(`/${baseBranch}`));
+};