refactor(core): make provider optional when use_target is set

Targets with use_target delegate to another target and don't need
their own provider. Removed redundant provider: mock from delegation
targets in root targets.yaml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: christso

.agentv/targets.yaml

@@ -13,19 +13,16 @@ targets:
   # Example: AGENT_TARGET=copilot-cli  or  AGENT_TARGET=claude
   - name: default
     use_target: ${{ AGENT_TARGET }}
-    provider: mock
 
   # ── LLM target (text generation, no agent binary needed) ────────────
   # Delegates to GRADER_TARGET — same provider used for grading and LLM evals.
   - name: llm
     use_target: ${{ GRADER_TARGET }}
-    provider: mock
 
   # ── Grader (LLM-as-judge) ──────────────────────────────────────────
   # Used by agent targets via grader_target. Switch provider via GRADER_TARGET.
   - name: grader
     use_target: ${{ GRADER_TARGET }}
-    provider: mock
 
   # ── Named agent targets ───────────────────────────────────────────
   - name: copilot-cli

packages/core/src/evaluation/providers/targets.ts

@@ -638,7 +638,7 @@ export const COMMON_TARGET_SETTINGS = [
 const BASE_TARGET_SCHEMA = z
   .object({
     name: z.string().min(1, 'target name is required'),
-    provider: z.string().min(1, 'provider is required'),
+    provider: z.string().optional(),
     use_target: z.string().optional(),
     grader_target: z.string().optional(),
     judge_target: z.string().optional(), // backward compat
@@ -719,6 +719,11 @@ export function resolveTargetDefinition(
       `${parsed.name}: target-level workspace_template has been removed. Use eval-level workspace.template.`,
     );
   }
+  if (!parsed.provider) {
+    throw new Error(
+      `${parsed.name}: 'provider' is required (targets with use_target must be resolved before calling resolveTargetDefinition)`,
+    );
+  }
   const provider = resolveString(
     parsed.provider,
     env,

packages/core/src/evaluation/providers/types.ts

@@ -282,7 +282,7 @@ export type EnvLookup = Readonly<Record<string, string | undefined>>;
 
 export interface TargetDefinition {
   readonly name: string;
-  readonly provider: ProviderKind | string;
+  readonly provider?: ProviderKind | string;
   // Delegation: resolve this target as another named target.
   // Supports ${{ ENV_VAR }} syntax (e.g., use_target: ${{ AGENT_TARGET }}).
   readonly use_target?: string | unknown | undefined;