The agent was telling users to run '/gsd:transition' after phase completion,
but this command does not exist. transition.md is an internal workflow invoked
by execute-phase during auto-advance.
Changes:
- Add <internal_workflow> banner to transition.md declaring it is NOT a user command
- Add explicit warning in execute-phase completion section that /gsd:transition
does not exist
- Add 'only suggest commands listed above' guard to prevent hallucination
- Update resume-project.md to avoid ambiguous 'Transition' label
- Replace 'ready for transition' with 'ready for next step' in execute-plan.md
Fixes#1081
