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/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
    */