feat: render the destination sections at init

init.build() can now write [misp], [abusedb] and [urlhaus], the three
sections report reads, so a user no longer has to hand-edit the config
for them. Follows the [reporter] pattern: a section is emitted only
when something was answered for it, never with an empty api_key.

A half-answered pair (misp_url given, misp_api_key skipped, or vice
versa) is rejected rather than silently written: config.load() would
classify it "incomplete" and report would refuse, but only after the
user believed setup succeeded. Task 8's prompts are the normal route
in, but build() is pure and a caller can hand-build a dict, so this is
enforced here too.

Also fixes FOOTER, which still promised a single [vendors] table; the
spec settled three separate sections and [misp] is no longer "a later
part" since this plan builds it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0176FYdVfpzUq8S9jecqQqL6

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