Fix render failing on fresh bare clones

[milldr]

What

• Call Fetch() after BareClone() in Render() to create remote tracking refs (refs/remotes/origin/*)
• Call EnsureRemoteRef() for the base branch before creating new-branch worktrees in Render()

Why

git clone --bare doesn't create refs/remotes/origin/* refs, and git fetch --all on a bare repo only fetches to FETCH_HEAD. When Render() tried to create a worktree with origin/main as the start point, it failed with fatal: invalid reference: origin/main. The Sync() method already handled this by calling EnsureRemoteRef(), but Render() did not.