Posibrain Fixes (#22196)

closes #21982
closes #21705

<img width="465" height="394" alt="image"
src="https://github.com/user-attachments/assets/6ae35510-73c5-46e8-b624-a83e71dca603"
/>

This PR fixes the longstanding IPC med rework bug whereby surgically
removing a posibrain would just delete it. Also fixes a bug where it
wasn't possible to join as a posibrain ghost role.

code/modules/organs/internal/species/machine/posibrain.dm

@@ -140,6 +140,7 @@
 		stored_mmi.forceMove(get_turf(src))
 		if(owner.mind)
 			owner.mind.transfer_to(stored_mmi.brainmob)
+		stored_mmi = null
 
 	. = ..()