refactor: nest Claude Code commands under BMad subdirectory (#307)

- Update installer config to use .claude/commands/BMad/ path
- Modify setupClaudeCode function to create nested directory structure
- Update documentation and upgrader to reflect new command location
- Improves organization by grouping all BMad commands together

tools/installer/config/install.config.yaml

@@ -21,7 +21,7 @@ ide-configurations:
       # 3. The agent will adopt that persona for the conversation
   claude-code:
     name: Claude Code
-    rule-dir: .claude/commands/
+    rule-dir: .claude/commands/BMad/
     format: multi-file
     command-suffix: .md
     instructions: |