fix: revert bmb module-definition path to src/module.yaml (#2146)

bmad-builder reverted its skills/ directory back to src/ for installer
compatibility (bmad-code-org/bmad-builder#40). Update the external
modules manifest to match.

Author: bmadcode

tools/installer/external-official-modules.yaml

@@ -4,7 +4,7 @@
 modules:
   bmad-builder:
     url: https://github.com/bmad-code-org/bmad-builder
-    module-definition: skills/module.yaml
+    module-definition: src/module.yaml
     code: bmb
     name: "BMad Builder"
     description: "Agent and Builder"