feat(skill): Phase 3 MP 기반 스킬 시스템 구현

- Skill, SkillType, BuffEffect, SkillState, SkillUseResult 클래스 정의 (skill.dart)
- SkillSystemState를 GameState에 통합 (activeBuffs, skillStates, elapsedMs)
- 프로그래밍 테마 스킬 데이터 정의 (skill_data.dart)
  - 공격: Debug Strike, Memory Leak, Core Dump, Kernel Panic 등
  - 회복: Hot Reload, Garbage Collection, Quick Fix
  - 버프: Safe Mode, Overclock, Firewall
- SkillService 구현 (skill_service.dart)
  - 스킬 사용 가능 여부 확인 (MP, 쿨타임)
  - 공격/회복/버프 스킬 사용 로직
  - 자동 스킬 선택 (HP < 30% → 회복, 보스전 → 강력한 공격, 일반 → MP 효율)
  - MP 자연 회복 (비전투: 50ms당 1, 전투: WIS 기반)
- progress_service.dart에 스킬 시스템 통합
  - tick()에서 스킬 시간 업데이트 및 버프 만료 처리
  - _processCombatTickWithSkills()로 전투 중 자동 스킬 사용

lib/src/core/model/game_state.dart

@@ -4,6 +4,7 @@ import 'package:askiineverdie/src/core/model/combat_state.dart';
 import 'package:askiineverdie/src/core/model/equipment_item.dart';
 import 'package:askiineverdie/src/core/model/equipment_slot.dart';
 import 'package:askiineverdie/src/core/model/item_stats.dart';
+import 'package:askiineverdie/src/core/model/skill.dart';
 import 'package:askiineverdie/src/core/util/deterministic_random.dart';
 
 /// Minimal skeletal state to mirror Progress Quest structures.
@@ -20,6 +21,7 @@ class GameState {
     SpellBook? spellBook,
     ProgressState? progress,
     QueueState? queue,
+    SkillSystemState? skillSystem,
   }) : rng = DeterministicRandom.clone(rng),
        traits = traits ?? Traits.empty(),
        stats = stats ?? Stats.empty(),
@@ -27,7 +29,8 @@ class GameState {
        equipment = equipment ?? Equipment.empty(),
        spellBook = spellBook ?? SpellBook.empty(),
        progress = progress ?? ProgressState.empty(),
-       queue = queue ?? QueueState.empty();
+       queue = queue ?? QueueState.empty(),
+       skillSystem = skillSystem ?? SkillSystemState.empty();
 
   factory GameState.withSeed({
     required int seed,
@@ -38,6 +41,7 @@ class GameState {
     SpellBook? spellBook,
     ProgressState? progress,
     QueueState? queue,
+    SkillSystemState? skillSystem,
   }) {
     return GameState(
       rng: DeterministicRandom(seed),
@@ -48,6 +52,7 @@ class GameState {
       spellBook: spellBook,
       progress: progress,
       queue: queue,
+      skillSystem: skillSystem,
     );
   }
 
@@ -60,6 +65,9 @@ class GameState {
   final ProgressState progress;
   final QueueState queue;
 
+  /// 스킬 시스템 상태 (Phase 3)
+  final SkillSystemState skillSystem;
+
   GameState copyWith({
     DeterministicRandom? rng,
     Traits? traits,
@@ -69,6 +77,7 @@ class GameState {
     SpellBook? spellBook,
     ProgressState? progress,
     QueueState? queue,
+    SkillSystemState? skillSystem,
   }) {
     return GameState(
       rng: rng ?? DeterministicRandom.clone(this.rng),
@@ -79,6 +88,76 @@ class GameState {
       spellBook: spellBook ?? this.spellBook,
       progress: progress ?? this.progress,
       queue: queue ?? this.queue,
+      skillSystem: skillSystem ?? this.skillSystem,
+    );
+  }
+}
+
+/// 스킬 시스템 상태 (Phase 3)
+///
+/// 스킬 쿨타임, 활성 버프, 게임 경과 시간 등을 관리
+class SkillSystemState {
+  const SkillSystemState({
+    required this.skillStates,
+    required this.activeBuffs,
+    required this.elapsedMs,
+  });
+
+  /// 스킬별 쿨타임 상태
+  final List<SkillState> skillStates;
+
+  /// 현재 활성화된 버프 목록
+  final List<ActiveBuff> activeBuffs;
+
+  /// 게임 진행 경과 시간 (밀리초, 스킬 쿨타임 계산용)
+  final int elapsedMs;
+
+  factory SkillSystemState.empty() => const SkillSystemState(
+    skillStates: [],
+    activeBuffs: [],
+    elapsedMs: 0,
+  );
+
+  /// 특정 스킬 상태 가져오기
+  SkillState? getSkillState(String skillId) {
+    for (final state in skillStates) {
+      if (state.skillId == skillId) return state;
+    }
+    return null;
+  }
+
+  /// 버프 효과 합산 (동일 버프는 중복 적용 안 됨)
+  ({double atkMod, double defMod, double criMod, double evasionMod}) get totalBuffModifiers {
+    double atkMod = 0;
+    double defMod = 0;
+    double criMod = 0;
+    double evasionMod = 0;
+
+    final seenBuffIds = <String>{};
+    for (final buff in activeBuffs) {
+      if (seenBuffIds.contains(buff.effect.id)) continue;
+      seenBuffIds.add(buff.effect.id);
+
+      if (!buff.isExpired(elapsedMs)) {
+        atkMod += buff.effect.atkModifier;
+        defMod += buff.effect.defModifier;
+        criMod += buff.effect.criRateModifier;
+        evasionMod += buff.effect.evasionModifier;
+      }
+    }
+
+    return (atkMod: atkMod, defMod: defMod, criMod: criMod, evasionMod: evasionMod);
+  }
+
+  SkillSystemState copyWith({
+    List<SkillState>? skillStates,
+    List<ActiveBuff>? activeBuffs,
+    int? elapsedMs,
+  }) {
+    return SkillSystemState(
+      skillStates: skillStates ?? this.skillStates,
+      activeBuffs: activeBuffs ?? this.activeBuffs,
+      elapsedMs: elapsedMs ?? this.elapsedMs,
     );
   }
 }