fix(config): route providers through unified config

[christso]

Summary

• rename the root provider catalog from .agentv/targets.yaml to .agentv/providers.yaml and reference it from .agentv/config.yaml with providers: file://providers.yaml
• make eval selection use the config-provided provider catalog path by default, with explicit --providers still overriding
• allow bare provider arrays for provider catalog refs and align config validation with project execution defaults
• update the readme quickstart example to the unified providers/defaults shape

Validation

• bun test packages/core/test/evaluation/providers/targets-file.test.ts packages/core/test/evaluation/loaders/config-loader.test.ts packages/core/test/evaluation/validation/config-validator.test.ts apps/cli/test/commands/eval/targets.test.ts apps/cli/test/commands/eval/run-command-options.test.ts
• bun --filter @agentv/core typecheck && bun --filter agentv typecheck
• bun apps/cli/src/cli.ts validate .agentv/config.yaml .agentv/providers.yaml
• bun apps/cli/src/cli.ts validate examples/features/readme-quickstart/.agentv/config.yaml examples/features/readme-quickstart/.agentv/providers.yaml examples/features/readme-quickstart/.agentv/defaults.yaml
• live dogfood with local OpenAI-compatible proxy: readme-quickstart passed 1/1 from example cwd and from repo root using root .agentv/config.yaml + .agentv/providers.yaml

Bead: av-yxx7

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