refactor: Simplify template output filenames

This commit simplifies the output filenames for the templates in the `bmad-wechat-mini-game-dev` expansion pack, as requested by the user.

The following templates have been updated:
- `game-brief-tmpl.yaml` now outputs `docs/brief.md`.
- `prd-tmpl.yaml` now outputs `docs/prd.md`.
- `game-design-doc-tmpl.yaml` now outputs `docs/gdd.md`.
- `game-architecture-tmpl.yaml` now outputs `docs/architecture.md`.

This change makes the filenames more direct and predictable.

expansion-packs/bmad-wechat-mini-game-dev/templates/game-architecture-tmpl.yaml

@@ -5,7 +5,7 @@ template:
   version: 1.0
   output:
     format: markdown
-    filename: "docs/{{game_name}}-game-architecture.md"
+    filename: "docs/architecture.md"
     title: "{{game_title}} WeChat Mini-Game Architecture Document"
 
 workflow:

expansion-packs/bmad-wechat-mini-game-dev/templates/game-brief-tmpl.yaml

@@ -5,7 +5,7 @@ template:
   version: 1.0
   output:
     format: markdown
-    filename: "docs/{{game_name}}-game-brief.md"
+    filename: "docs/brief.md"
     title: "{{game_title}} Game Brief"
 
 workflow:

expansion-packs/bmad-wechat-mini-game-dev/templates/game-design-doc-tmpl.yaml

@@ -5,7 +5,7 @@ template:
   version: 1.0
   output:
     format: markdown
-    filename: "docs/{{game_name}}-game-design-document.md"
+    filename: "docs/gdd.md"
     title: "{{game_title}} Game Design Document (GDD)"
 
 workflow: