fix: clarify the value of SkillMetadata.path (#12729)

Rename `SkillMetadata.path` to `SkillMetadata.path_to_skills_md` for
clarity.

Would ideally change the type to `AbsolutePathBuf`, but that can be done
later.

codex-rs/core/src/mcp/skill_dependencies.rs

@@ -434,7 +434,7 @@ mod tests {
             dependencies: Some(SkillDependencies { tools }),
             policy: None,
             permissions: None,
-            path: PathBuf::from("skill"),
+            path_to_skills_md: PathBuf::from("skill"),
             scope: SkillScope::User,
         }
     }