dispatch and apply_commands returned English prose that became the reply
subject, so the pure layers owned author-facing copy. They now return
reason::Reason and the mail-building edge resolves it through the catalogue.
core::rules needed no change: RuleError was already a typed enum, and its
Display stays for author-side tools. The daemon maps the variants itself
rather than borrowing that prose.
Error-path tests now assert on variants, so rewording main.ftl cannot break
them, and PruneModeUnknown carries the value the author actually sent.
Reason::every_variant_resolves_to_copy walks the enum, so a variant added
without a catalogue entry fails in tests rather than mailing someone
"???reason-…???".
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