chore: Adds AI instructions and SKILLs for ModernUO codebase (#2347)

Summary

  - Adds CLAUDE.md at repo root with 14 terse code audit rules (always loaded, low token cost)
  - Adds pointer files for other AI tools: AGENTS.md (Codex), GEMINI.md, .github/COPILOT-INSTRUCTIONS.md (Copilot), .cursorrules (Cursor) — all redirect to CLAUDE.md as single source of truth
  - Gitignores /.claude so personal AI config isn't distributed
  - Moves Claude skills to dev-docs/claude-skills/ (opt-in, not auto-loaded)
  - Adds 14 dev-docs covering codebase conventions

  Code Audit Rules (in CLAUDE.md)

  1. LINQ tiered rules (Tier 1 free, Tier 2 warm, Tier 3 forbidden)
  2. No Console.WriteLine — use LogFactory.GetLogger()
  3. No concurrency primitives in game code
  4. No World.Mobiles/World.Items iteration
  5. Clean up refs in OnDelete()/OnAfterDelete()
  6. Cancel timers in OnDelete()/OnAfterDelete()
  7. STArrayPool<T>.Shared not ArrayPool<T>.Shared
  8. PooledRefList<T> not new List<T>() on hot paths
  9. Serialization: partial class, [Constructible], no serialized TimerExecutionToken
  10. No Task.Run/new Thread() in game code
  11. Never assume era — ask which expansion
  12. _camelCase fields, PascalCase properties/methods
  13. No empty gumps — use DisplayTo() pattern
  14. PropertyList string literals must be {} holes, cliloc-as-argument uses :#

dev-docs/claude-skills/modernuo-era-expansion.md

@@ -0,0 +1,134 @@
+---
+name: modernuo-era-expansion
+description: >
+  Trigger when writing era-conditional code, using Core.AOS/SE/ML etc., or when user hasn't specified target era. Always ask which expansion to target if not specified.
+---
+
+# ModernUO Era & Expansion Support
+
+## When This Activates
+- Writing code that depends on game era (damage formulas, skill caps, mechanics)
+- Using `Core.AOS`, `Core.SE`, `Core.ML`, etc.
+- User asks for a feature without specifying era
+- Implementing mechanics that changed across expansions
+
+## CRITICAL RULE
+**Never assume era.** If the user hasn't specified which expansion to target, ASK before writing era-dependent code.
+
+## Expansion Enum
+
+```csharp
+public enum Expansion
+{
+    None,    // 0 - Pre-T2A
+    T2A,     // 1 - The Second Age
+    UOR,     // 2 - Renaissance
+    UOTD,    // 3 - Third Dawn
+    LBR,     // 4 - Blackthorn's Revenge
+    AOS,     // 5 - Age of Shadows (major overhaul)
+    SE,      // 6 - Samurai Empire
+    ML,      // 7 - Mondain's Legacy
+    SA,      // 8 - Stygian Abyss
+    HS,      // 9 - High Seas
+    TOL,     // 10 - Time of Legends
+    EJ       // 11 - Endless Journey
+}
+```
+
+## Era Check Properties
+
+Each returns `true` when `Core.Expansion >= that era`:
+
+```csharp
+Core.T2A   // >= The Second Age
+Core.UOR   // >= Renaissance
+Core.UOTD  // >= Third Dawn
+Core.LBR   // >= Blackthorn's Revenge
+Core.AOS   // >= Age of Shadows
+Core.SE    // >= Samurai Empire
+Core.ML    // >= Mondain's Legacy
+Core.SA    // >= Stygian Abyss
+Core.HS    // >= High Seas
+Core.TOL   // >= Time of Legends
+Core.EJ    // >= Endless Journey
+```
+
+For exact expansion: `Core.Expansion == Expansion.AOS`
+
+## Key Era Boundaries
+
+### AOS (Age of Shadows) -- Most Significant Change
+- Complete combat overhaul: resistance-based damage system
+- Property-based item system (magic properties)
+- New damage formula: `GetNewAosDamage()` vs flat `Utility.Random()`
+- Luck system for loot
+- Insurance system
+
+### SE (Samurai Empire)
+- Bushido / Ninjitsu skills
+- Samurai/Ninja classes
+- Loot pack adjustments
+
+### ML (Mondain's Legacy)
+- Spellweaving
+- Adjusted skill gain chances
+- Container weight display changes
+
+## Pattern: Era-Conditional Code
+
+```csharp
+// Ternary for simple values
+var delay = Core.SE ? 250 : Core.AOS ? 500 : 1000;
+
+// If/else for logic branches
+if (Core.AOS)
+{
+    damage = GetNewAosDamage(10, 1, 4, target);
+}
+else
+{
+    damage = Utility.Random(4, 4);
+    if (CheckResisted(target))
+        damage *= 0.75;
+}
+
+// Property display varies by era
+if (Core.ML)
+{
+    list.Add(1072241, $"{TotalItems}\t{MaxItems}\t{TotalWeight}\t{MaxWeight}");
+}
+else
+{
+    list.Add(1050044, $"{TotalItems}\t{TotalWeight}");
+}
+```
+
+## Pattern: Era-Dependent Loot
+
+`LootPack` properties auto-select based on expansion:
+```csharp
+// These automatically pick the right era variant
+LootPack.Poor       // OldPoor / AosPoor / SePoor
+LootPack.Average    // OldAverage / AosAverage / SeAverage
+LootPack.Rich       // OldRich / AosRich / SeRich
+LootPack.FilthyRich // OldFilthyRich / AosFilthyRich / SeFilthyRich
+LootPack.UltraRich  // OldUltraRich / AosUltraRich / SeUltraRich
+```
+
+## Anti-Patterns
+
+- **Hardcoding mechanics for one era** without conditional checks
+- **Assuming AOS** when the user might want pre-AOS
+- **Using era-specific APIs** without checking (e.g., `GetNewAosDamage()` pre-AOS)
+
+## Real Examples
+- Era-conditional damage: `Projects/UOContent/Spells/First/MagicArrow.cs`
+- Era-conditional properties: `Projects/Server/Items/Container.cs` (`GetProperties`)
+- Era-conditional values: `Projects/UOContent/Items/Weapons/Ranged/BaseRanged.cs`
+- Expansion enum: `Projects/Server/ExpansionInfo.cs`
+- Stat config by era: `Projects/UOContent/Skills/SkillCheck.cs`
+
+## See Also
+- `dev-docs/era-expansion.md` - Complete expansion documentation
+- `dev-docs/claude-skills/modernuo-content-patterns.md` - Content templates
+- `dev-docs/claude-skills/modernuo-configuration.md` - Configuration system