fix(walkthrough): use remote default branch

packages/web/server/lib/git/DOCUMENTATION.md

@@ -105,6 +105,7 @@ The following functions are internal helpers used by exported functions:
 - `ahead`: Number of commits ahead of upstream.
 - `behind`: Number of commits behind upstream.
 - `upstreamComparison`: Optional comparison against `upstream/<current-branch>`, with `{ remote, branch, ahead, behind }`.
+- `defaultBranches`: Remote default branches derived from local symbolic refs such as `remotes/origin/HEAD -> origin/main`, keyed by remote name. Omitted by runtimes that do not provide this Git metadata.
 - `files`: Array of file objects with `path`, `index`, `working_dir` status codes.
 - `isClean`: Boolean indicating if working tree is clean.
 - `diffStats`: Object mapping file paths to `{ insertions, deletions }`.

packages/web/server/lib/git/service.js

@@ -3367,6 +3367,7 @@ export async function getBranches(directory) {
     const allBranches = result.all;
     const remoteBranches = allBranches.filter(branch => branch.startsWith('remotes/'));
     const activeRemoteBranches = await filterActiveRemoteBranches(git, remoteBranches);
+    const defaultBranches = await getRemoteDefaultBranches(git);
 
     const filteredAll = [
       ...allBranches.filter(branch => !branch.startsWith('remotes/')),
@@ -3376,7 +3377,8 @@ export async function getBranches(directory) {
     return {
       all: filteredAll,
       current: result.current,
-      branches: result.branches
+      branches: result.branches,
+      defaultBranches,
     };
   } catch (error) {
     console.error('Failed to get branches:', error);
@@ -3384,6 +3386,28 @@ export async function getBranches(directory) {
   }
 }
 
+async function getRemoteDefaultBranches(git) {
+  try {
+    const refs = await git.raw([
+      'for-each-ref',
+      '--format=%(refname) %(symref)',
+      'refs/remotes',
+    ]);
+    return Object.fromEntries(
+      refs.trim().split('\n').flatMap((line) => {
+        const [ref, symbolicRef] = line.split(' ');
+        const match = ref.match(/^refs\/remotes\/([^/]+)\/HEAD$/);
+        const prefix = match ? `refs/remotes/${match[1]}/` : '';
+        return match && typeof symbolicRef === 'string' && symbolicRef.startsWith(prefix)
+          ? [[match[1], symbolicRef.slice(prefix.length)]]
+          : [];
+      })
+    );
+  } catch {
+    return {};
+  }
+}
+
 async function filterActiveRemoteBranches(git, remoteBranches) {
   try {
     const remotes = await git.getRemotes();

packages/web/server/lib/git/service.test.js

@@ -10,6 +10,7 @@ import {
   cherryPick,
   createWorktree,
   getWorktreeBootstrapStatus,
+  getBranches,
   getStatus,
   isGitRepository,
   populateWorktreeWithLockRecovery,
@@ -988,3 +989,25 @@ describe('hash validation', () => {
     ).rejects.not.toThrow('Invalid commit hash');
   });
 });
+
+describe.runIf(canRunGit())('getBranches', () => {
+  it('returns a remote default branch whose name is not a conventional fallback', async () => {
+    const remote = createTempDir();
+    const repository = createTempDir();
+    runGit(remote, ['init', '--bare', '--initial-branch=react']);
+    runGit(repository, ['init', '-b', 'next']);
+    runGit(repository, ['config', 'user.email', 'test@example.com']);
+    runGit(repository, ['config', 'user.name', 'Test']);
+    fs.writeFileSync(path.join(repository, 'README.md'), '# Test\n');
+    runGit(repository, ['add', 'README.md']);
+    runGit(repository, ['commit', '-m', 'init']);
+    runGit(repository, ['remote', 'add', 'origin', remote]);
+    runGit(repository, ['push', 'origin', 'HEAD:react']);
+    runGit(repository, ['fetch', 'origin']);
+    runGit(repository, ['remote', 'set-head', 'origin', '--auto']);
+
+    await expect(getBranches(repository)).resolves.toMatchObject({
+      defaultBranches: { origin: 'react' },
+    });
+  });
+});

packages/web/server/lib/walkthrough/DOCUMENTATION.md

@@ -55,6 +55,11 @@ written against staged code never silently re-anchors onto an unstaged edit.
 | `branch` | `branch` | `getRangeDiff` uses three-dot `base...head`, so work merged in from the base branch is excluded |
 | `pr` | `pr:<number>` | GitHub returns the merge-base diff, matching the branch semantics |
 
+For the current-branch source, the UI prefers the default branch of the current
+branch's tracking remote (from its local `remote/HEAD` symbolic ref), then uses
+the existing conventional-branch fallback. It does not offer the source when the
+chosen base cannot be resolved locally or through a remote-tracking ref.
+
 The panel offers the current branch's pull request on its own: it registers with
 the shared GitHub PR status store (`useGitHubPrStatusStore`) rather than waiting
 for the pull request panel to have been visited. That store already dedupes