fix(enable): address review findings on the storage question

From /review, four review agents, and Copilot (all converging):

• A cancelled storage prompt no longer persists the recommendation silently: it prints 'Using the recommended git-refs checkpoint storage.' Every other cancelled setup prompt skips its action, so adopting one here must be disclosed (silent-failure review, HIGH).
• A cancelled command context (SIGINT/SIGTERM) now stops enable instead of being swallowed as a form cancellation and mutating the repo further (silent-failure review, HIGH).
• New subtest covers the headless-without---yes first run — the most common real path, previously only exercised with Yes: true.
• Comment accuracy: 'silently' no longer claimed for the cancelled case (Copilot + comment review); 'pre-selected' attribution and the 'matching optional prompts' claim made precise.
• strings import moved to the stdlib group (Copilot + code review).
• Architecture docs no longer imply refs is unconditional for new setups; it is the recommended, pre-selected choice.