Enable evals CI: add ANTHROPIC_API_KEY secret

[siracusa5]

The evals workflow skips silently when ANTHROPIC_API_KEY is not set (see #ci: skip evals gracefully). Once harness-kit is further along, enable it:

Steps

1. Add the secret:

   gh secret set ANTHROPIC_API_KEY --repo harnessprotocol/harness-kit
   

2. Verify the next release run picks it up and the compatibility page updates automatically (evals/results/latest.json + website/docs/evals/compatibility.md).

Context

Evals run on every release publish and commit updated results back to the repo. The workflow is in .github/workflows/evals.yml. Nothing else needs to change — just the secret.