/pr <anything> silently resolved to /prompt via the shortest-match
tiebreaker in prefix expansion, permanently overwriting the system
prompt and persisting to config. The command's functionality (setting
agent.system_prompt) is available via config.yaml and /personality
covers the common use case.
Removes: CommandDef, dispatch branch, _handle_prompt_command handler,
docs references, and updates subcommand extraction test.
2772d99085