[BUG] Phantom PR branch shown in UI that was never created

Bug description

When starting a Claude Code session, the system prompt includes a Main branch field like:

Main branch (you will usually use this for PRs): claude/some-random-id

This branch does not actually exist — neither locally nor on the remote. Running git branch -a confirms it's absent. However, it appears in the UI as a suggested PR target (e.g., claude/some-id ← main), which is confusing because it implies the branch was already created.

Steps to reproduce

1. Open a project with Claude Code (git repo on main branch)
2. Note the Main branch value in the session header
3. Run git branch -a — the branch does not exist
4. The UI still shows it as if it's a real branch with a PR relationship

Expected behavior

Either:

• Don't show a branch name until it's actually created
• Or clearly label it as "suggested branch name" rather than displaying it as an existing branch

Environment

• Claude Code CLI
• macOS (Darwin, ARM64)
• Git repository with only main branch