Remove standalone promptfoo suite in favor of the existing eval harness

Drops the parallel setup added in 4335dc7 (apps/cli/ai/tests/promptfoo + .github/workflows/prompt-eval.yml). Studio already has an agent eval suite at eval/promptfoo.config.yaml that runs the real agent via startAiAgent() and a Studio-authed grader; the new tests will live there.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: youknowriad