fix(sandbox): allowlist example placeholders for detect-secrets

Fixes failing CI check: tekton/code-detect-secrets

The example env file snippet in sandbox/README.md tripped IBM's
detect-secrets scanner. Mark the obvious placeholders with
`# pragma: allowlist secret` so they pass while remaining readable.

Author: vinodmut

sandbox/README.md

@@ -64,10 +64,10 @@ model):
 
 ```bash
 # Direct Anthropic API
-ANTHROPIC_API_KEY=sk-ant-...
+ANTHROPIC_API_KEY=sk-ant-xxxx  # pragma: allowlist secret
 
 # Or, via a proxy / gateway
-ANTHROPIC_AUTH_TOKEN=your-token
+ANTHROPIC_AUTH_TOKEN=your-token  # pragma: allowlist secret
 ANTHROPIC_BASE_URL=https://your-gateway.example.com
 CLAUDE_MODEL=claude-sonnet-4-6
 CLAUDE_CODE_SKIP_BEDROCK_AUTH=1