feat(mentor): show co-mentors on workspace page (PR8 Task 9)

- Adds mentor.getProjectMentors({ projectId }) — returns all active
  MentorAssignment rows for a project, authorized to any mentor on it
- Workspace page header surfaces "You + N co-mentor(s): names…" so each
  mentor knows the team composition without having to ask the admin

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(mentor)/mentor/workspace/[projectId]/page.tsx

@@ -1,21 +1,29 @@
 'use client'
 
 import { useParams, useRouter } from 'next/navigation'
+import { useSession } from 'next-auth/react'
 import { trpc } from '@/lib/trpc/client'
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card'
 import { Button } from '@/components/ui/button'
 import { Badge } from '@/components/ui/badge'
 import { Skeleton } from '@/components/ui/skeleton'
 import { Tabs, TabsContent, TabsList, TabsTrigger } from '@/components/ui/tabs'
+import {
+  Tooltip,
+  TooltipContent,
+  TooltipProvider,
+  TooltipTrigger,
+} from '@/components/ui/tooltip'
 import { WorkspaceChat } from '@/components/mentor/workspace-chat'
 import { FilePromotionPanel } from '@/components/mentor/file-promotion-panel'
 import { WorkspaceFilesPanel } from '@/components/mentor/workspace-files-panel'
-import { ArrowLeft, MessageSquare, FileText, Upload } from 'lucide-react'
+import { ArrowLeft, MessageSquare, FileText, Upload, Users } from 'lucide-react'
 import { toast } from 'sonner'
 
 export default function MentorWorkspaceDetailPage() {
   const params = useParams()
   const router = useRouter()
+  const { data: session } = useSession()
   const projectId = params.projectId as string
 
   // Get mentor assignment for this project
@@ -27,6 +35,22 @@ export default function MentorWorkspaceDetailPage() {
     { enabled: !!projectId }
   )
 
+  // Co-mentor visibility (PR8 multi-mentor): show who else is on the team.
+  // Gracefully tolerates stale tabs where the caller no longer has access
+  // (assignment dropped) — query just returns nothing in that case.
+  const { data: projectMentors } = trpc.mentor.getProjectMentors.useQuery(
+    { projectId },
+    { enabled: !!projectId, retry: false }
+  )
+
+  const currentUserId = session?.user?.id
+  const coMentors = (projectMentors ?? []).filter(
+    a => a.mentor.id !== currentUserId
+  )
+  const coMentorNames = coMentors.map(a => a.mentor.name ?? 'Unnamed mentor')
+  const visibleCoMentors = coMentorNames.slice(0, 3)
+  const hiddenCoMentors = coMentorNames.slice(3)
+
   if (isLoading) {
     return (
       <div className="space-y-6">
@@ -70,6 +94,37 @@ export default function MentorWorkspaceDetailPage() {
           {project.teamName && (
             <p className="text-muted-foreground mt-1">{project.teamName}</p>
           )}
+          {coMentors.length > 0 && (
+            <div className="mt-2 flex items-center gap-1.5 text-sm text-muted-foreground">
+              <Users className="h-3.5 w-3.5 shrink-0" />
+              <span>
+                You + {coMentors.length} co-mentor
+                {coMentors.length === 1 ? '' : 's'}:{' '}
+                <span className="text-foreground">
+                  {visibleCoMentors.join(', ')}
+                </span>
+                {hiddenCoMentors.length > 0 && (
+                  <>
+                    {' '}
+                    <TooltipProvider>
+                      <Tooltip>
+                        <TooltipTrigger asChild>
+                          <span className="cursor-help underline decoration-dotted underline-offset-2">
+                            +{hiddenCoMentors.length} more
+                          </span>
+                        </TooltipTrigger>
+                        <TooltipContent>
+                          <div className="text-xs">
+                            {hiddenCoMentors.join(', ')}
+                          </div>
+                        </TooltipContent>
+                      </Tooltip>
+                    </TooltipProvider>
+                  </>
+                )}
+              </span>
+            </div>
+          )}
         </div>
       </div>
 

src/server/routers/mentor.ts

@@ -1326,6 +1326,50 @@ export const mentorRouter = router({
     return assignments
   }),
 
+  /**
+   * List all active mentors assigned to a project (PR8 multi-mentor).
+   *
+   * Returns one row per active MentorAssignment (droppedAt = null) with the
+   * mentor's id + name. Used by the mentor workspace page to display the
+   * co-mentor team so each mentor knows who else they're working with.
+   *
+   * Authorization: caller must be an active mentor on the project (or an
+   * admin via mentorProcedure). Non-assigned mentors get FORBIDDEN.
+   */
+  getProjectMentors: mentorProcedure
+    .input(z.object({ projectId: z.string() }))
+    .query(async ({ ctx, input }) => {
+      const isAdmin = ['SUPER_ADMIN', 'PROGRAM_ADMIN'].includes(ctx.user.role)
+
+      if (!isAdmin) {
+        const ownAssignment = await ctx.prisma.mentorAssignment.findFirst({
+          where: {
+            projectId: input.projectId,
+            mentorId: ctx.user.id,
+            droppedAt: null,
+          },
+          select: { id: true },
+        })
+        if (!ownAssignment) {
+          throw new TRPCError({
+            code: 'FORBIDDEN',
+            message: 'You are not assigned to mentor this project',
+          })
+        }
+      }
+
+      const assignments = await ctx.prisma.mentorAssignment.findMany({
+        where: { projectId: input.projectId, droppedAt: null },
+        select: {
+          id: true,
+          mentor: { select: { id: true, name: true } },
+        },
+        orderBy: { assignedAt: 'asc' },
+      })
+
+      return assignments
+    }),
+
   /**
    * Get detailed project info for a mentor's assigned project
    */