Document hidden behaviors and improve CLI help discoverability

[johnjosephhorton]

Severity: low-medium.

• source.known_options fallback for option-less context questions (cli.py:context_question_options).
• Rank auto-detection rules, so users can name/word columns to trigger them (see the rank-detection enhancement).
• Positional context-question selection (see the context-selection enhancement).
• That twin-validate / the baseline need the key in the actual environment until the .env-loading bug is fixed.
• Top-level zwill --help lists every leaf command flat in one {...} block; the grouped subcommands (survey/raw/question/respondent/answer/…) aren't discoverable from the top. Group them, or add a "command overview" section.

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Found while building a digital-twin validation for a 33-respondent SMB survey (smb.csv) end-to-end via zwill guide (survey → import → commit → one-shot marginals → twin-probability job → twin-validate → report). File/line refs are from zwill/ at that checkout.