Add t.Context() guideline to CLAUDE.md

Document that tests should use t.Context() instead of context.Background()
as Go 1.21+ provides test context that is automatically cancelled when
the test ends.

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

Author: vojtabiberle

CLAUDE.md

@@ -146,6 +146,7 @@ Services auto-reload on code changes using Air.
 - Coverage target: 80%
 - E2E tests use real etcd and mocked HTTP clients
 - Test projects configured via `.env` file pointing to `projects.json`
+- **Use `t.Context()` instead of `context.Background()`** - Go 1.21+ provides test context via `t.Context()` which is automatically cancelled when the test ends
 
 ## State Management Architecture