fix: prevent extension command shadowing (#1994)

* fix: prevent extension command shadowing

* Validate extension command namespaces

* Reuse extension command name pattern

extensions/template/extension.yml

@@ -47,8 +47,8 @@ provides:
     - name: "speckit.my-extension.example"
       file: "commands/example.md"
       description: "Example command that demonstrates functionality"
-      # Optional: Add aliases for shorter command names
-      aliases: ["speckit.example"]
+      # Optional: Add aliases in the same namespaced format
+      aliases: ["speckit.my-extension.example-short"]
 
     # ADD MORE COMMANDS: Copy this block for each command
     # - name: "speckit.my-extension.another-command"