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.

src/plugin/tool-registry.ts

@@ -113,7 +113,7 @@ export function createToolRegistry(args: {
       }
     : {}
 
-  const hashlineEnabled = pluginConfig.experimental?.hashline_edit ?? true
+  const hashlineEnabled = pluginConfig.hashline_edit ?? true
   const hashlineToolsRecord: Record<string, ToolDefinition> = hashlineEnabled
     ? { edit: createHashlineEditTool() }
     : {}