Simplify workspace creation with inline worktree API

Replace the multi-step new-agent route flow with a single
create_paseo_worktree endpoint that registers the workspace immediately
and creates the git worktree in the background. The sidebar now calls
this endpoint directly and shows a creating spinner inline.

Also auto-resolves the base branch from origin/HEAD when not explicitly
provided, removes the unused Tauri WebSocket transport layer, and
normalizes loopback endpoints to localhost.

Closes #125

packages/app/src/components/left-sidebar.tsx

@@ -618,7 +618,8 @@ const styles = StyleSheet.create((theme) => ({
     alignItems: 'center',
     gap: theme.spacing[2],
     paddingVertical: theme.spacing[1],
-    paddingHorizontal: theme.spacing[1],
+    paddingRight: theme.spacing[1],
+    paddingLeft: theme.spacing[3],
     flexShrink: 0,
   },
   newAgentButtonHovered: {},