New Forge::rewrite + PassKind::Rewrite. An author re-authors
existing chapter prose entirely in their voice — sentence rhythm,
word choice, paragraph shape all become theirs — while canon
(names, dates, places, events, order, technical facts) is preserved
exactly. Not editing; re-authoring. SystemMode::Replace, max effort.
skald rewrite --chapter <uuid> [--author slug] overwrites body_md
with the rewritten version. The pre-rewrite prose is stashed in the
new chapters.body_md_original column on first rewrite (migration
0008, idempotent) so the original is never lost. body_md_tts is
cleared — it was annotated against the old prose and must be
regenerated by a fresh prepare-narration.
prepare-narration gains --single-voice: skips the character speaker
roster so no [voice:X] dialogue tags are inserted, only beat
markers. Right for one-voice narration.
Migration 0008 also extends generation_runs.kind to allow 'rewrite'.
