fix: Standardize environment variable names (KRX_API_KEY → KRX_OPENAPI, OPENDART_API_KEY → OPENDART_API)

- Updated KrxDataService.cs: Environment.GetEnvironmentVariable("KRX_API_KEY") → KRX_OPENAPI
- Updated OpenDartService.cs: OPENDART_API_KEY → OPENDART_API
- Updated Program.cs: ResolveSecret() calls with new env var names
- Updated tests/OpenDartServiceTests.cs: Test fixture environment variable
- Updated CLAUDE.md: Documentation with corrected env var names
- Verified: 95/95 integration tests PASS (stub data mode, no API keys required)
- AGENTS.md v16.0 compliance: Explicit environment variable resolution

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Modules.ModelOperations/ShadowRun/Services/KrxDataService.cs

@@ -159,11 +159,11 @@ public sealed class KrxDataService : IKrxDataService
         CancellationToken cancellationToken)
     {
         // Real KRX OpenAPI: Stock Price endpoint
-        var apiKey = Environment.GetEnvironmentVariable("KRX_API_KEY") ?? "";
+        var apiKey = Environment.GetEnvironmentVariable("KRX_OPENAPI") ?? "";
 
         if (string.IsNullOrEmpty(apiKey))
         {
-            _logger.LogWarning("KRX_API_KEY not set, using stub data");
+            _logger.LogWarning("KRX_OPENAPI not set, using stub data");
             // Fallback to stub for local development (KRX format)
             await Task.Delay(100, cancellationToken);
             return $$"""