adding abilities, created skill tree template and unlock mechanics

api/app/data/abilities/plague.yaml

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+# Plague - Necromancer Dark Affliction ability
+# Spreading poison DoT
+
+ability_id: "plague"
+name: "Plague"
+description: "Infect your target with a virulent plague that spreads to nearby enemies"
+ability_type: "spell"
+base_power: 15
+damage_type: "poison"
+scaling_stat: "intelligence"
+scaling_factor: 0.5
+mana_cost: 20
+cooldown: 3
+is_aoe: true
+target_count: 0
+effects_applied:
+  - effect_id: "plagued"
+    name: "Plagued"
+    effect_type: "dot"
+    duration: 4
+    power: 8
+    stat_affected: null
+    stacks: 1
+    max_stacks: 3
+    source: "plague"