chore: remove agent-evals workflow and evaluation code (#10989)

### Description
Removes the deprecated GitHub Actions agent-evals.yaml workflow, the agent_evals_build step in node-test.yml, the scripts/agent-evals package and templates, and related configuration references in ignore files and documentation.

### Scenarios Tested
- Verified build: npm run build
- Verified linting: npm run lint:changed-files
- Verified unit tests: npm run mocha:fast (5,151 passing)

### Sample Commands
N/A