Add Prism Launcher (Minecraft) to first-boot flatpaks

Microsoft account sign-in documented in OPERATIONS.md; Prism manages
its own Java runtimes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

OPERATIONS.md

@@ -74,6 +74,9 @@ high clocks. Change one thing at a time and re-check `sensors`.
   `bc250-remote-setup` hasn't been run yet.
 - Steam misbehaves after an update → try ProtonUp-Qt (installed) to switch
   the game to Proton-GE.
+- Minecraft: open Prism Launcher, add an account (Microsoft sign-in — a paid
+  Minecraft: Java Edition license on that account), then create an instance
+  for the version/modpack you want. Prism downloads its own Java.
 - GeForce NOW (beta) installs itself on first boot with internet. The BC-250
   has no hardware video decode, so the stream decodes on the CPU: keep it at
   1080p (higher settings will stutter and run hot). If it's missing, check