feat: Decisions page — aggregated view of all decision points across traces

Adds /dashboard/decisions page with colored type badges, search,
filters (by type), sort (newest/oldest/costliest), pagination,
and links to parent traces. New /api/decisions endpoint with
Prisma queries. Removes 'Soon' badge from sidebar nav.

apps/web/src/app/dashboard/layout.tsx

@@ -8,7 +8,6 @@ import {
   GitBranch,
   Settings,
   Menu,
-  X,
   ChevronRight,
 } from "lucide-react";
 import { cn } from "@/lib/utils";
@@ -22,7 +21,7 @@ interface NavItem {
 
 const navItems: NavItem[] = [
   { href: "/dashboard", label: "Traces", icon: Activity },
-  { href: "/dashboard/decisions", label: "Decisions", icon: GitBranch, comingSoon: true },
+  { href: "/dashboard/decisions", label: "Decisions", icon: GitBranch },
   { href: "/dashboard/settings", label: "Settings", icon: Settings, comingSoon: true },
 ];