fix(config): promote hashline_edit to top-level flag

Move hashline_edit out of experimental so it is a stable top-level config with default-on runtime behavior and explicit disable support. Add migration and tests to preserve existing experimental.hashline_edit users without breaking configs.

assets/oh-my-opencode.schema.json

@@ -128,6 +128,9 @@
         "type": "string"
       }
     },
+    "hashline_edit": {
+      "type": "boolean"
+    },
     "agents": {
       "type": "object",
       "properties": {
@@ -3141,9 +3144,6 @@
         },
         "disable_omo_env": {
           "type": "boolean"
-        },
-        "hashline_edit": {
-          "type": "boolean"
         }
       },
       "additionalProperties": false