Base changeling objective(s) (#39562)

Co-authored-by: slarticodefast <161409025+slarticodefast@users.noreply.github.com>

Resources/Prototypes/GameRules/roundstart.yml

@@ -232,6 +232,8 @@
   - type: GameRule
     minPlayers: 25
   - type: AntagSelection
+    agentName: changeling-round-end-agent-name
+    selectionTime: IntraPlayerSpawn
     definitions:
     - prefRoles: [ Changeling ]
       max: 3
@@ -249,6 +251,9 @@
         - ActionRetractableItemArmBlade # Temporary addition, will inevitably be a purchasable in the bio-store
       mindRoles:
       - MindRoleChangeling
+  - type: AntagObjectives
+    objectives:
+    - ChangelingSurviveObjective
 
 - type: entity
   id: Revolutionary