refactor(hashline): override native edit tool instead of separate tool + disabler hook

Replace 3-component hashline system (separate hashline_edit tool + edit
disabler hook + OpenAI-exempted read enhancer) with 2-component system
that directly overrides the native edit tool key, matching the
delegate_task pattern.

- Register hashline tool as 'edit' key to override native edit
- Delete hashline-edit-disabler hook (no longer needed)
- Delete hashline-provider-state module (no remaining consumers)
- Remove OpenAI exemption from read enhancer (explicit opt-in means all providers)
- Remove setProvider wiring from chat-params

src/plugin/tool-registry.ts

@@ -120,7 +120,7 @@ export function createToolRegistry(args: {
 
   const hashlineEnabled = pluginConfig.experimental?.hashline_edit ?? false
   const hashlineToolsRecord: Record<string, ToolDefinition> = hashlineEnabled
-    ? { hashline_edit: createHashlineEditTool() }
+    ? { edit: createHashlineEditTool() }
     : {}
 
   const allTools: Record<string, ToolDefinition> = {