fix: use ANTHROPIC_AUTH_TOKEN for Claude CLI env injection

- Change env var from ANTHROPIC_API_KEY to ANTHROPIC_AUTH_TOKEN
- Add Backend field propagation in taskSpec (cli.go)
- Add stderr logging for injected env vars with API key masking
- Add comprehensive tests for env injection flow

Generated with SWE-Agent.ai

Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/internal/backend/claude.go

@@ -25,7 +25,7 @@ func (ClaudeBackend) Env(baseURL, apiKey string) map[string]string {
 		env["ANTHROPIC_BASE_URL"] = baseURL
 	}
 	if apiKey != "" {
-		env["ANTHROPIC_API_KEY"] = apiKey
+		env["ANTHROPIC_AUTH_TOKEN"] = apiKey
 	}
 	return env
 }