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>

tests/KArtSell.Integration.Tests/OpenDartServiceTests.cs

@@ -15,8 +15,8 @@ public class OpenDartServiceTests : IAsyncLifetime
     {
         _dataSource = fixture.DataSource;
         // Set test API key to avoid initialization error
-        if (string.IsNullOrEmpty(Environment.GetEnvironmentVariable("OPENDART_API_KEY")))
-            Environment.SetEnvironmentVariable("OPENDART_API_KEY", "test-key-12345");
+        if (string.IsNullOrEmpty(Environment.GetEnvironmentVariable("OPENDART_API")))
+            Environment.SetEnvironmentVariable("OPENDART_API", "test-key-12345");
         _service = new OpenDartService(_dataSource, new HttpClient(), fixture.Clock(), fixture.Logger<OpenDartService>());
     }