fix: stop init duplicating its own preamble on every rewrite

The carry-across preserved every line before the first [table], but that
text is build()'s own header, not the user's. A second init emitted it
twice and a third three times, with [general] missing from the tail half,
so the file grew a copy per run. It still parsed, which is why the suite
stayed green and only a hand test found it.

Preserving from the first table onward fixes the trailing "Later parts"
block too: it trails whichever table came last, so it is already inside a
skipped section. A separate guard for it passed its own mutation check
and was removed as redundant.

Required prompts now say so. The relay question and the hop picker cannot
be skipped, because with no trust boundary parse refuses and init is the
route out, but they read like the reporter questions that do offer a skip.
The cases prompt says "Enter for <default>" rather than showing the
default in brackets, which was the same ambiguity in the other direction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LByBnw83xr9YP85nskzkyE

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