Add skills.roots config for explicit skill roots

Add skills.roots to config so users can specify absolute skill folders.
Use configured roots as the local root set when provided; otherwise keep default local roots.
Add loader/config tests and regenerate the committed config schema fixture.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/config.schema.json

@@ -1545,6 +1545,13 @@
             "$ref": "#/definitions/SkillConfig"
           },
           "type": "array"
+        },
+        "roots": {
+          "description": "Additional absolute skill root directories to scan for `SKILL.md`.\n\nWhen non-empty, these roots override default local skill roots.",
+          "items": {
+            "$ref": "#/definitions/AbsolutePathBuf"
+          },
+          "type": "array"
         }
       },
       "type": "object"