[BUG] Moon CI resolves nonexistent master branch

Platform

GitHub Actions (ubuntu-latest, windows-latest, macos-latest) and Windows 11 locally with Moon 2.3.3.

Current Behavior

Every Bun CI run on main fails before a project task starts. The latest run, https://github.com/OpenCut-app/OpenCut/actions/runs/29121395671, fails in moon ci with:

fatal: ambiguous argument 'master': unknown revision or path not in the working tree.

.moon/workspace.yml does not define vcs.defaultBranch, so Moon 2.3.3 falls back to master, but this repository's default branch is main.

Expected Behavior

moon ci should compare against main and proceed to the affected task graph.

Steps To Reproduce

1. Clone the repository at bab8af831b354a0b5a98a4a6e818ab7d633b94df.
2. Run bunx --bun @moonrepo/cli@2.3.3 ci from the repository root.
3. Observe the failure resolving master.

Proposed Fix

Add the following to .moon/workspace.yml:

vcs:
  defaultBranch: 'main'

This is limited to Moon's affected-change base resolution. It does not alter application runtime behavior, GitHub's default branch, or task definitions.

I reproduced the current failure locally. With the explicit main base, Moon resolves the task graph without the master error. The current contribution policy asks for maintainer approval before a bug-fix PR, so requesting approval for this one-line CI correction.

Start with .moon/workspace.yml and reproduce the failure with bunx --bun @moonrepo/cli@2.3.3 ci from the repository root. Set Moon's VCS default branch to main, then rerun the command and confirm it resolves the affected task graph without the nonexistent master-branch error.