cowork-bot: fix DELETE_BEFORE_CREATE cost estimation + deepcopy pricing defaults

- Remove DELETE_BEFORE_CREATE from the zero-after-cost guard so
  replacements report the new resource cost instead of $0.
- Use copy.deepcopy for DEFAULT_PRICICING to prevent global state
  mutation from custom pricing loads leaking across calls/tests.
- Add regression test for delete-before-create after cost.

Author: DevForge Engineer

src/deploydiff/cost_estimator.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import copy
 import json
 from pathlib import Path
 
@@ -199,10 +200,7 @@ def _estimate_resource_cost(
     # If deleting, after cost is 0; if creating, before cost is 0
     if before and change.action == ChangeAction.CREATE:
         return 0.0
-    if not before and change.action in (
-        ChangeAction.DELETE,
-        ChangeAction.DELETE_BEFORE_CREATE,
-    ):
+    if not before and change.action == ChangeAction.DELETE:
         return 0.0
 
     resource_type = change.resource_type
@@ -251,7 +249,7 @@ def _load_pricing(
         custom = json.load(f)
 
     # Merge with defaults (custom overrides)
-    merged = DEFAULT_PRICING.copy()
+    merged = copy.deepcopy(DEFAULT_PRICING)
     for resource_type, prices in custom.items():
         if resource_type in merged:
             merged[resource_type].update(prices)

tests/test_edge_cases.py

@@ -16,7 +16,7 @@
 from rich.console import Console
 
 from deploydiff.cli import _load_plan, _render_costs
-from deploydiff.cost_estimator import DEFAULT_PRICING, _load_pricing
+from deploydiff.cost_estimator import DEFAULT_PRICING, _load_pricing, estimate_costs
 from deploydiff.models import (
     ChangeAction,
     ChangeSource,
@@ -94,6 +94,25 @@ def test_load_pricing_custom_type_not_in_defaults(self, tmp_path):
         # Defaults should still be present
         assert "t3.micro" in pricing["aws_instance"]
 
+    def test_delete_before_create_has_nonzero_after_cost(self):
+        """DELETE_BEFORE_CREATE should report the new resource cost as after_cost."""
+        pricing = _load_pricing()
+        change = ResourceChange(
+            address="aws_instance.replaced",
+            action=ChangeAction.DELETE_BEFORE_CREATE,
+            resource_type="aws_instance",
+            resource_name="replaced",
+            source=ChangeSource.TERRAFORM,
+            before={"instance_type": "t3.micro"},
+            after={"instance_type": "t3.large"},
+        )
+        plan = DeployPlan(source=ChangeSource.TERRAFORM, changes=[change])
+        estimates = estimate_costs(plan)
+        assert len(estimates) == 1
+        est = estimates[0]
+        assert est.monthly_cost_before == pricing["aws_instance"]["t3.micro"]
+        assert est.monthly_cost_after == pricing["aws_instance"]["t3.large"]
+
 
 class TestRollbackEdgeCases:
     """Tests for uncovered rollback paths."""