feat: Sprint 4 — scenario planner, report builder, comments, dashboard widgets

What-If Scenario Planner (G5):
- New /projects/[id]/scenario page with side-by-side baseline vs scenario
- simulate mutation: pure cost/hours/headcount/utilization computation
- apply mutation: creates real PROPOSED assignments from scenario
- Impact cards: cost delta, hours delta, headcount, skill coverage %
- Per-resource utilization impact table with over-allocation warnings
- "What-If" button added to project detail page

Custom Report Builder (G7):
- New /reports/builder page with full config panel
- Entity selector (resource/project/assignment), column picker, filter builder
- Dynamic Prisma query with eq/neq/gt/lt/contains/in operators
- Sortable results table with pagination (50/page)
- CSV export via exportReport mutation
- Sidebar nav link under Analytics

Collaboration Layer (G8):
- Comment model in Prisma (entityType/entityId, replies, @mentions, resolved)
- comment router: list, count, create, resolve, delete
- @mention parsing with notification creation + SSE delivery
- CommentInput with @mention autocomplete (arrow nav, Enter/Tab confirm)
- CommentThread with avatar, timestamp, reply, resolve, delete
- Integrated as "Comments" tab in estimate workspace with count badge

Dashboard Widgets:
- BudgetForecastWidget: progress bars per project, burn rate, exhaustion date
- SkillGapWidget: supply vs demand per skill, shortage/surplus indicators
- ProjectHealthWidget: 3-dimension health circles + composite score
- 3 new application use-cases + dashboard router queries
- All registered in widget-registry with lazy imports

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/app/(app)/projects/[id]/page.tsx

@@ -84,6 +84,16 @@ export default async function ProjectDetailPage({ params }: ProjectDetailPagePro
               </div>
               <div className="mt-0.5 flex items-center">Win probability: {project.winProbability}%<InfoTooltip content="Likelihood of winning this project (0-100%). Used to calculate weighted pipeline value (budget x probability)." /></div>
             </div>
+            <Link
+              href={`/projects/${id}/scenario`}
+              className="inline-flex items-center gap-2 rounded-lg border border-indigo-300 bg-white px-3 py-2 text-sm font-medium text-indigo-700 shadow-sm hover:bg-indigo-50 transition dark:border-indigo-600 dark:bg-gray-800 dark:text-indigo-300 dark:hover:bg-indigo-900/20"
+              title="Open What-If Scenario Planner"
+            >
+              <svg className="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M8 9l3 3-3 3m5 0h3M5 20h14a2 2 0 002-2V6a2 2 0 00-2-2H5a2 2 0 00-2 2v12a2 2 0 002 2z" />
+              </svg>
+              What-If
+            </Link>
             <ProjectDetailActions project={project as never} />
           </div>
         </div>