refactor(usage): simplify-pass follow-ups

- Single-source the included note as _INCLUDED_NOTE and attach it at
  BOTH status='included' sites (the zero-amount pricing-entry branch
  previously returned the same status with no note).
- Docstring/comment precision on format_cost_label: the fallback
  triggers on 4dp ROUNDING to 0.0000 (banker's rounding includes the
  exact $0.00005 boundary), not truncation; note why the rendered-label
  guard beats a naive Decimal threshold.
- Tests: replaced a dead assertion with the exact-boundary case
  ($0.00005), fixed an overclaiming comment, aligned the terminal
  cost column.

agent/insights.py

@@ -1022,7 +1022,7 @@ class InsightsEngine:
             lines.append("  💰 Cost")
             lines.append("  " + "─" * 56)
             if est_cost > 0:
-                lines.append(f"  Estimated:         {_fmt_est_cost(est_cost)}")
+                lines.append(f"  Estimated:          {_fmt_est_cost(est_cost)}")
             if included_sessions > 0:
                 lines.append(
                     f"  Included:           {included_sessions} session(s) "

agent/usage_pricing.py

@@ -22,14 +22,20 @@ _NOUS_DEFAULT_BASE_URL = "https://inference-api.nousresearch.com/v1"
 # the display is non-zero (e.g. $0.0046 instead of $0.00). See #79220.
 _SUBCENT_THRESHOLD = Decimal("0.01")
 
+# Attached to every CostResult with status="included" so consumers can
+# distinguish "free because subscription" from "free because $0 pricing".
+_INCLUDED_NOTE = "subscription-included; no provider invoice for usage"
+
 
 def format_cost_label(amount: Decimal) -> str:
     """Format a cost amount as a display label.
 
     Scales precision to magnitude:
     - Zero → "$0.00"
-    - Sub-cent (< $0.01) → "~$0.0046" (4 dp; below $0.00005 falls back
-      to "~$<0.0001" so the label never reads as zero)
+    - Sub-cent (< $0.01) → "~$0.0046" (4 dp; amounts that ROUND to
+      0.0000 at 4 dp — i.e. at or below $0.00005 under banker's
+      rounding — fall back to "~$<0.0001" so the label never reads
+      as zero)
     - Normal → "~$1.23" (2 dp)
 
     This fixes #79220 where sub-cent per-turn costs on cheap models
@@ -44,8 +50,11 @@ def format_cost_label(amount: Decimal) -> str:
         return "$0.00"
     if amount < _SUBCENT_THRESHOLD:
         label = f"~${amount:.4f}"
-        # 4dp truncation of a positive amount below $0.00005 would render
+        # A positive amount that rounds to 0.0000 at 4 dp would render
         # "~$0.0000" — a zero-looking label, the exact #79220 dishonesty.
+        # Comparing the rendered label checks the truth directly (a naive
+        # `< 0.00005` threshold misses the exact boundary under
+        # ROUND_HALF_EVEN).
         return label if label != "~$0.0000" else "~$<0.0001"
     return f"~${amount:.2f}"
 
@@ -1365,7 +1374,7 @@ def estimate_usage_cost(
             source="none",
             label="included",
             pricing_version="included-route",
-            notes=("subscription-included; no provider invoice for usage",),
+            notes=(_INCLUDED_NOTE,),
         )
 
     entry = get_pricing_entry(model_name, provider=provider, base_url=base_url, api_key=api_key)
@@ -1414,6 +1423,7 @@ def estimate_usage_cost(
     if entry.source == "none" and amount == _ZERO:
         status = "included"
         label = "included"
+        notes.append(_INCLUDED_NOTE)
 
     if route.provider == "openrouter":
         notes.append("OpenRouter cost is estimated from the models API until reconciled.")

tests/agent/test_insights.py

@@ -701,7 +701,8 @@ class TestEdgeCases:
         report = engine.generate(days=30)
         text = engine.format_terminal(report)
 
-        # The cost section should appear with all three buckets
+        # The cost section should appear with the buckets this DB has
+        # (estimated + included; no unknown-cost session is created here)
         assert "💰 Cost" in text
         assert "~$1.50" in text  # estimated
         assert "included" in text.lower()

tests/agent/test_usage_pricing.py

@@ -416,7 +416,9 @@ class TestFormatCostLabel:
         """
         label = format_cost_label(Decimal("0.00004"))
         assert label == "~$<0.0001"
-        assert "0.0000" not in label.replace("<0.0001", "")
+        # Exact boundary: $0.00005 rounds to 0.0000 under ROUND_HALF_EVEN
+        # and must also take the fallback.
+        assert format_cost_label(Decimal("0.00005")) == "~$<0.0001"
 
     def test_sub_cent_deepseek_scenario(self):
         """Reproduce the #79220 reproduction: DeepSeek at $0.004640."""