Add model_id to CostMetadata

Tracks which LLM powered each research call. Enables:
- Cost analysis across model tiers
- Quality calibration (confidence vs model capability)
- Reproducibility (know exactly what produced a result)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

ResearchContract.md

@@ -311,22 +311,30 @@ class CostMetadata:
     iterations_run: int       # Number of inner-loop iterations
     wall_time_sec: float      # Actual elapsed time
     budget_exhausted: bool    # True if researcher hit iteration or token cap
+    model_id: str             # Model used for the research loop (e.g. "claude-sonnet-4-6")
 ```
 
+The `model_id` field records which LLM powered the researcher's inner loop. This is critical for:
+- **Cost analysis** — comparing token spend across model tiers
+- **Quality calibration** — correlating confidence scores with model capability
+- **Reproducibility** — knowing exactly what produced a given result
+
 Example:
 ```python
 CostMetadata(
     tokens_used=8452,
     iterations_run=3,
     wall_time_sec=42.5,
-    budget_exhausted=False
+    budget_exhausted=False,
+    model_id="claude-sonnet-4-6"
 )
 ```
 
 The PI uses this to:
-- Track costs (token budgets, actual spend)
+- Track costs (token budgets, actual spend, model tier)
 - Detect runaway loops (budget_exhausted = True)
 - Plan timeouts (wall_time_sec tells you if this is acceptable latency)
+- Compare fidelity-to-cost ratio across models
 
 ---
 
@@ -430,7 +438,8 @@ Response:
     "tokens_used": 450,
     "iterations_run": 1,
     "wall_time_sec": 3.2,
-    "budget_exhausted": false
+    "budget_exhausted": false,
+    "model_id": "claude-sonnet-4-6"
   },
   "trace_id": "550e8400-e29b-41d4-a716-446655440001"
 }
@@ -494,7 +503,8 @@ Response:
     "tokens_used": 19240,
     "iterations_run": 4,
     "wall_time_sec": 67.8,
-    "budget_exhausted": false
+    "budget_exhausted": false,
+    "model_id": "claude-sonnet-4-6"
   },
   "trace_id": "550e8400-e29b-41d4-a716-446655440002"
 }
@@ -562,7 +572,8 @@ Response:
     "tokens_used": 4998,
     "iterations_run": 3,
     "wall_time_sec": 31.2,
-    "budget_exhausted": true
+    "budget_exhausted": true,
+    "model_id": "claude-haiku-4-5"
   },
   "trace_id": "550e8400-e29b-41d4-a716-446655440003"
 }