adding abilities, created skill tree template and unlock mechanics

api/app/data/abilities/execute.yaml

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+# Execute - Vanguard Weapon Master ability
+# Bonus damage to low HP targets
+
+ability_id: "execute"
+name: "Execute"
+description: "Finish off weakened enemies. Deals bonus damage to targets below 30% HP"
+ability_type: "attack"
+base_power: 60
+damage_type: "physical"
+scaling_stat: "strength"
+scaling_factor: 0.6
+mana_cost: 40
+cooldown: 3
+is_aoe: false
+target_count: 1
+effects_applied: []