fix(core): reject authored use_target targets

[christso]

Summary

Authored targets.yaml entries now fail validation when they use use_target, including env-template forms such as use_target: "{{ env.AGENT_TARGET }}". This turns the previous validate/pass then prepare/eval failure into an immediate removed-field validation error.

Generic unknown fields remain warning-only by default. That keeps AgentV aligned with Promptfoo's permissive validation behavior for unrecognized fields while still treating use_target as a targeted unsupported field in AgentV 5 authored targets.

Validation

• bun test packages/core/test/evaluation/validation/targets-validator.test.ts
• bun run build
• bun apps/cli/src/cli.ts validate /tmp/tmp.dii994QnHC/use/.agentv/targets.yaml confirmed use_target exits non-zero.
• bun apps/cli/src/cli.ts validate /tmp/tmp.dii994QnHC/warn/.agentv/targets.yaml confirmed unknown provider settings remain warnings with exit 0.
• bun apps/cli/src/cli.ts validate /tmp/tmp.dii994QnHC/project/.agentv/config.yaml confirmed unexpected config fields remain warnings with exit 0.
• bun apps/cli/src/cli.ts validate /tmp/tmp.dii994QnHC/warn/.agentv/targets.yaml --max-warnings 0 confirmed warnings can still be promoted to failure.
• bunx biome check packages/core/src/evaluation/providers/targets.ts packages/core/src/evaluation/validation/targets-validator.ts packages/core/test/evaluation/validation/targets-validator.test.ts

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