feat: git forest branch -l compatibility

## Summary
- `git forest branch` should mirror `git branch` — listing branches, creating branches, etc.
- `git forest branch -l` should work like `git branch -l`, showing all trees with `*`/`+` prefixes
- This is the natural git-compatible way to list trees, complementing `git forest status`

## Context
The status command already uses `git branch -l` conventions (`*` for active, `+` for worktrees, default branch first). A `git forest branch` subcommand would make this discoverable via the familiar git interface.