refactor(editor): extract zone-pack orchestrators into cli_zone_packs.cpp

Continues the modularization started in 6c9ab6fa. Moves the four
pack-orchestrator handlers (--gen-zone-texture-pack, -mesh-pack,
-starter-pack, --gen-project-starter-pack) into their own file
following the same handle<Family> pattern.

Side cleanup:
  - Consolidated the duplicated --seed flag parser into a single
    parseSeedFlag helper
  - Consolidated the std::system + > /dev/null wrap into runSilently

main.cpp drops 28,329 → 28,070 lines (-259). Pack family is fully
self-contained (~260 lines), behavior unchanged (verified by
re-running gen-zone-starter-pack and confirming 6 PNGs + 5 WOMs).

tools/editor/cli_zone_packs.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+namespace wowee {
+namespace editor {
+namespace cli {
+
+// Dispatch the four zone/project pack-orchestrator handlers:
+//   --gen-zone-texture-pack
+//   --gen-zone-mesh-pack
+//   --gen-zone-starter-pack
+//   --gen-project-starter-pack
+//
+// Each fans out to per-asset commands via subprocess, so this
+// module has no dependency on the procedural generators it calls.
+//
+// Returns true if matched; outRc holds the exit code. Returns
+// false if no match — caller should continue its dispatch chain.
+bool handleZonePacks(int& i, int argc, char** argv, int& outRc);
+
+} // namespace cli
+} // namespace editor
+} // namespace wowee