feat: TypeScript targets config (agentv.config.ts) #908

Problem

Target configuration in targets.yaml is growing complex with use_target delegation, env var interpolation (${{ VAR }}), and provider-specific fields. YAML lacks type safety, autocomplete, and conditional logic.

Proposal

Extend the existing agentv.config.ts (via defineConfig()) to support targets alongside eval config:

import { defineConfig } from 'agentv';

export default defineConfig({
  targets: {
    default: { use_target: process.env.AGENT_TARGET ?? 'copilot-cli' },
    grader: { use_target: process.env.GRADER_TARGET ?? 'openrouter' },
    llm: { use_target: process.env.GRADER_TARGET ?? 'openrouter' },
    'copilot-cli': {
      provider: 'copilot-cli',
      model: process.env.COPILOT_MODEL ?? 'gpt-5-mini',
      grader_target: 'grader',
    },
    openrouter: {
      provider: 'openrouter',
      api_key: process.env.OPENROUTER_API_KEY!,
      model: process.env.OPENROUTER_MODEL ?? 'openai/gpt-5.4-mini',
    },
  },
});

Benefits over YAML

Prior art

• Vitest: vitest.config.ts replaced JSON config
• Next.js: next.config.ts replaced next.config.js
• ESLint: eslint.config.ts flat config
• promptfoo: promptfooconfig.ts alongside YAML
• Tailwind: tailwind.config.ts

All major frameworks are converging on TypeScript config for complex configuration.

Backward compatibility

• targets.yaml continues to work unchanged
• agentv.config.ts targets take precedence when both exist
• Discovery order: agentv.config.ts > .agentv/targets.yaml