refactor: simplify module discovery to scan entire project

- Module discovery now scans entire project recursively for install-config.yaml
- Removed hardcoded module locations (bmad-custom-src, etc.)
- Modules can exist anywhere with _module-installer/install-config.yaml
- All modules treated equally regardless of location
- No special UI handling for 'custom' modules
- Core module excluded from selection list (always installed first)
- Only install-config.yaml is valid (removed support for legacy config.yaml)

Modules are now discovered by structure, not location.

.gitignore

@@ -62,7 +62,7 @@ src/modules/bmm/sub-modules/
 src/modules/bmb/sub-modules/
 src/modules/cis/sub-modules/
 src/modules/bmgd/sub-modules/
-
+shared-modules
 z*/
 
 .bmad