feat: add New Goal dialog and Sub Goal button

- Added NewGoalDialog component with title, description (markdown),
  status, level, and parent goal selection
- Integrated dialog into DialogContext with parentId defaults support
- Added "+ New Goal" button on /goals page (both empty state and header)
- Added "+ Sub Goal" button on goal detail sub-goals tab that pre-fills
  the parent goal

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

ui/src/components/Layout.tsx

@@ -6,6 +6,7 @@ import { PropertiesPanel } from "./PropertiesPanel";
 import { CommandPalette } from "./CommandPalette";
 import { NewIssueDialog } from "./NewIssueDialog";
 import { NewProjectDialog } from "./NewProjectDialog";
+import { NewGoalDialog } from "./NewGoalDialog";
 import { NewAgentDialog } from "./NewAgentDialog";
 import { OnboardingWizard } from "./OnboardingWizard";
 import { useDialog } from "../context/DialogContext";
@@ -84,6 +85,7 @@ export function Layout() {
       <CommandPalette />
       <NewIssueDialog />
       <NewProjectDialog />
+      <NewGoalDialog />
       <NewAgentDialog />
       <OnboardingWizard />
     </div>