Apply the ts-build-config (lib/types) convention to the new subagent packages

Master's #36 moved declaration output to lib/types (and types/exports/files
point there). The merge applied that to all pre-existing packages, but the
subagent backends introduced on this stack (subagent-inprocess, subagent-spawn,
subagent-fork) still used the old lib/ layout. Bring them onto the new
convention and add them to the single typecheck tsconfig.json references.

packages/subagent/subagent-spawn/tsconfig.json

@@ -2,7 +2,7 @@
   "extends": "../../../tsconfig.base.json",
   "compilerOptions": {
     "rootDir": "src",
-    "outDir": "lib"
+    "outDir": "lib/types"
   },
   "include": [
     "src"