wizard: catch and repair Sonstiges follow-ups without relevant (#47 follow-up)

[jstet]

A user imported a generated XLSForm into Kobo and found the conditional logic didn't work. Cause: every relevant column was empty. The model wrote follow-up text questions (Sonstiges fields, yes/no follow-ups) but never emitted the relevant expression that hides them from respondents who didn't pick the trigger choice. The validator can't catch this — it doesn't parse relevant — so the file passed and shipped with no skip-logic.

Three layers, complementary:

A. Prompt. Replace "Sonstiges + a follow-up text question with relevant logic instead of or_other" with a hard rule: the follow-up is required, name it <parent>_sonstiges / <parent>_other, and write the right expression (selected(${parent}, '<code>') for select_multiple parents, ${parent} = '<code>' for select_one / yes-no parents). Same rule covers yes/no follow-ups.

B. Quality check. New looksLikeFollowUp heuristic in qualityFeedback: a text question whose name ends in _sonstiges / _sonst / _other or whose label starts with "Falls Sonstiges" / "If other", with no relevant, is a broken follow-up. The gap goes into the repair loop; the repair agent fills in the expression.

C. Regression test. End-to-end: model emits a broken follow-up, assert the lead agent runs the repair loop and the shipped survey has the right relevant. Plus unit tests for the heuristic (name-suffix, German label prefix, English label prefix, no match, select_one ignored, already-has-relevant). Plus a new e2e fixture missing-relevant-on-follow-up documenting the failure mode in snapshots so future work can see what the validator currently misses.

Filed separately. The structural "dangling references and bad literals" check is filed upstream as CorrelAid/formtransform#73 — that's the long-term answer for non-formulaid users, and lands as a follow-up in formulaid once shipped.

Verified: npm run test:unit (100 pass, +9), npm run test:e2e (49 pass, +4), npm run check, npm run lint.

Closes #47 follow-up.