feat: Mentees & Activity tab on /admin/mentors

Adds a project-centric ops view for mentor management:
- New mentor.getMenteeActivity tRPC procedure aggregates every project
  with wantsMentorship=true and derives a status (unassigned / assigned
  / active / stalled) from the latest message + file activity.
- /admin/mentors becomes a tabbed page: existing Mentor list +
  new Mentees & Activity table with status pills, search, and a
  per-row Assign/Open CTA linking to /admin/projects/[id]/mentor.
- Includes 2 unit tests covering classification + program scoping.

Also: ignore .remember/ (plugin scratch dir).

.gitignore

@@ -62,3 +62,4 @@ build-output.txt
 # Private keys and secrets
 private/
 public/build-id.json
+.remember/