Document GitHub auto-delete setting to reduce merged branch clutter (#27)

Adds an explicit optional setup step for enabling GitHub's automatic head-branch deletion so merged agent branches are cleaned up remotely without manual pruning.

Constraint: Setting lives in GitHub repository settings, not local CLI config
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep this note in the setup checklist near branch workflow steps so users enable it during initial repo setup
Tested: README diff and checklist insertion verified locally
Not-tested: End-to-end GitHub settings toggle behavior (external UI)

Co-authored-by: NagyVikt <nagy.viktordp@gmail.com>

Author: NagyVikt

README.md

@@ -241,6 +241,9 @@ Use this exact checklist to setup multi-agent safety in this repository for Code
 7) Optional: sync your current agent branch with latest base branch:
    gx sync --check
    gx sync
+
+8) Optional (GitHub remote cleanup): enable:
+   Settings -> General -> Pull Requests -> Automatically delete head branches
 ```
 
 ## Basic commands