refactor(targets): use use_target for llm and grader targets

Both llm and grader now delegate via use_target: ${{ GRADER_TARGET }}
instead of hardcoding openrouter. Switch grader provider with one
env var: GRADER_TARGET=openrouter or GRADER_TARGET=gemini-llm.

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

Author: christso

.agentv/targets.yaml

@@ -16,16 +16,16 @@ targets:
     provider: mock
 
   # ── LLM target (text generation, no agent binary needed) ────────────
+  # Delegates to GRADER_TARGET — same provider used for grading and LLM evals.
   - name: llm
-    provider: openrouter
-    api_key: ${{ OPENROUTER_API_KEY }}
-    model: ${{ OPENROUTER_MODEL }}
+    use_target: ${{ GRADER_TARGET }}
+    provider: mock
 
   # ── Grader (LLM-as-judge) ──────────────────────────────────────────
+  # Used by agent targets via grader_target. Switch provider via GRADER_TARGET.
   - name: grader
-    provider: openrouter
-    api_key: ${{ OPENROUTER_API_KEY }}
-    model: ${{ OPENROUTER_MODEL }}
+    use_target: ${{ GRADER_TARGET }}
+    provider: mock
 
   # ── Named agent targets ───────────────────────────────────────────
   - name: copilot-cli