refactor(editor): extract --migrate-* handlers into cli_migrate.cpp

Moves the four schema-migration handlers (--migrate-wom,
--migrate-zone, --migrate-project, --migrate-jsondbc) out of
main.cpp into a new cli_migrate.{hpp,cpp} module. Same
behavior — they're idempotent in-place upgraders for older
WOM revisions and JSON DBC sidecar schemas. main.cpp shrinks
by 282 lines (11,261 → 10,979).

tools/editor/cli_migrate.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+namespace wowee {
+namespace editor {
+namespace cli {
+
+// Dispatch the schema-migration handlers — these upgrade older
+// on-disk asset versions in-place so newer tooling stops rejecting
+// or silently misreading them. All migrators are idempotent:
+// already-modern files become no-ops.
+//   --migrate-wom       single WOM v1/v2 → WOM3 (single-batch)
+//   --migrate-zone      every WOM in a zone directory
+//   --migrate-project   every zone in a project directory
+//   --migrate-jsondbc   JSON DBC sidecar schema fixes
+//
+// Returns true if matched; outRc holds the exit code.
+bool handleMigrate(int& i, int argc, char** argv, int& outRc);
+
+} // namespace cli
+} // namespace editor
+} // namespace wowee