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/config/schema/hooks.ts

@@ -45,7 +45,6 @@ export const HookNameSchema = z.enum([
   "tasks-todowrite-disabler",
   "write-existing-file-guard",
   "anthropic-effort",
-  "hashline-edit-disabler",
   "hashline-read-enhancer",
 ])