refactor: split main.ts into domain runtimes

changes/260-main-runtime-refactor.md

@@ -0,0 +1,6 @@
+type: internal
+area: main
+
+- Split `src/main.ts` into domain runtime wrappers and startup sequencing helpers.
+- Removed the last direct `src/main/runtime/*-main-deps.ts` imports from `src/main.ts`.
+- Kept startup behavior and IPC contracts stable while reducing composition-root size.