fix: add user-invocable: true to skill frontmatter (#2077)

Skills were missing this field, causing them to be treated as
"managed" instead of user-invocable via /speckit-* commands.

src/specify_cli/agents.py

@@ -275,7 +275,9 @@ class CommandRegistrar:
             },
         }
         if agent_name == "claude":
-            # Claude skills should only run when explicitly invoked.
+            # Claude skills should be user-invocable (accessible via /command)
+            # and only run when explicitly invoked (not auto-triggered by the model).
+            skill_frontmatter["user-invocable"] = True
             skill_frontmatter["disable-model-invocation"] = True
         return skill_frontmatter