[Bug]: Create PR fails with "base invalid" when the worktree base exists only locally#22920

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/desktop

Steps to reproduce

1. In a repository with a GitHub origin, create a branch from main and do not push it (for example a stack parent you have not pushed yet, or a leftover local branch). It exists only as refs/heads/<parent>.
2. Create a T3 worktree/thread with that branch as its base (baseRef: "<parent>", either through a new-worktree thread or t3_worktree_handoff). T3 records branch.<child>.gh-merge-base = <parent>.
3. Commit and push the child branch (git push -u origin <child>). The push succeeds.
4. Click Create PR in the thread's source-control panel.

Expected behavior

T3 should not send a base branch that the remote does not have. It should validate the base against refs/remotes/origin/<base> (or the GitHub API), and either fall back to the repository's default branch or show an actionable error naming the missing remote base.

Actual behavior

Create PR fails with:

The push itself succeeds; only PR creation fails. git config --get branch.<child>.gh-merge-base returns the local-only parent, while the remote has no such branch (gh api repos/<owner>/<repo>/branches/<parent> returns 404). Choosing main manually in the Create PR menu works.

Impact

Minor bug or occasional failure — it blocks PR creation until the base is corrected manually, but it is easy to hit whenever a worktree is created from a local-only or stale branch.

Version or commit

T3 Code (Beta) 0.1.1 (CFBundleShortVersionString 0.1.1)

Environment

macOS 27.2, GitHub cloud provider, SSH remote (git@github.com:...), desktop app.

Logs or stack traces

Not a crash. The full UI error is the createChangeRequest message above.

Relevant behaviour in the bundled 0.1.1 app:

• The worktree's baseRef ends up in branch.<child>.gh-merge-base; GitVcsDriver.pushCurrentBranch.recordMergeBase also writes that key when it is empty.
• PR base resolution reads branch.<child>.gh-merge-base and accepts it after a local branchExists(cwd, <base>) check, returning refs/heads/<base>.
• createChangeRequest forwards that value as baseBranch / baseRefName to the GitHub provider.

A base that exists only in the local clone therefore passes T3's check and is rejected by GitHub, so the user sees an opaque provider error instead of a base-selection problem.

Workaround

Point the merge base at a branch that exists on the remote:

or push the intended parent first (git push -u origin <parent>) and retry Create PR.