fix(bgm): resolve include patterns relative to package dir

The default include matched every yaml across the games root, so a
package could absorb a sibling game's defs (e.g. two game#main setups).
Resolve patterns against the package declaration's directory instead;
a leading slash marks a pattern as root-relative.

docs/bgm-format.md

@@ -104,6 +104,13 @@ package. **Defaults to `./**/*.yaml`**, so all yaml in the same and sub
 folders is discovered with no configuration. This also matches the package
 declaration itself, which is fine — it's the package, not a part.
 
+Patterns are resolved **relative to the package declaration's own directory**,
+not the games root. So a package declared in `carcassonne/carcassonne.md`
+with the default `./**/*.yaml` only picks up yaml under `carcassonne/` — it
+never absorbs defs from a sibling game. To reach outside its folder, a
+package can use a `../`-relative pattern or an absolute-from-root pattern
+(e.g. `**/shared/*.yaml`).
+
 ---
 
 ## 3. Roles