[codex] Document Jules branch and PR creation choices

[Hanjo92]

Summary

• Document create-session --branch as Jules API startingBranch.
• Add default, non-main, release, and existing feature branch selection guidance.
• Clarify when --automation-mode AUTO_CREATE_PR is an intended side effect and when to omit it.

Closes #14

Verification

• git diff --check
• python3 -m py_compile google-jules-control/scripts/jules_api.py
• python3 -m unittest discover -s tests
• rg check for branch, startingBranch, and AUTO_CREATE_PR guidance