adding abilities, created skill tree template and unlock mechanics

api/app/data/abilities/coup_de_grace.yaml

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+# Coup de Grace - Assassin Blade Specialist ability
+# Execute low HP targets
+
+ability_id: "coup_de_grace"
+name: "Coup de Grace"
+description: "Deliver the killing blow. Instantly kills targets below 25% HP, otherwise deals massive damage"
+ability_type: "attack"
+base_power: 70
+damage_type: "physical"
+scaling_stat: "dexterity"
+scaling_factor: 0.6
+mana_cost: 40
+cooldown: 4
+is_aoe: false
+target_count: 1
+effects_applied: []