feat: update_milestone tool and workspace milestone refresh

- Add update_milestone LLM tool: accepts project + milestone title to
  look up, then applies any of title/description/status changes
- WorkspaceView SSE watcher now triggers taskPanelRef.reload() on
  create_milestone and update_milestone success, so milestone groups
  appear immediately without a manual refresh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/WorkspaceView.vue

@@ -80,7 +80,7 @@ watch(
         activeNoteId.value = tc.result.data.id as number;
       }
       if (
-        ["create_task", "update_task"].includes(tc.function) &&
+        ["create_task", "update_task", "create_milestone", "update_milestone"].includes(tc.function) &&
         tc.status === "success"
       ) {
         taskPanelRef.value?.reload();