feat: add time-based pricing lookup for historical accuracy

Introduces LookupPricingAt() to resolve model pricing at a specific
timestamp instead of always using current prices. This is important
for accurate cost calculations when analyzing historical sessions
where pricing may have changed.

Changes:
- Add modelPricingVersion struct with EffectiveFrom timestamp
- Add defaultPricingHistory map for versioned pricing entries
- Update LookupPricing to delegate to LookupPricingAt(model, time.Now())
- Add comprehensive tests for time-windowed pricing lookup

The infrastructure supports future pricing changes by adding entries
to defaultPricingHistory with their effective dates.

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

internal/config/pricing_test.go

@@ -0,0 +1,81 @@
+package config
+
+import (
+	"testing"
+	"time"
+)
+
+func mustDate(t *testing.T, s string) time.Time {
+	t.Helper()
+	d, err := time.Parse("2006-01-02", s)
+	if err != nil {
+		t.Fatalf("parse date %q: %v", s, err)
+	}
+	return d
+}
+
+func TestLookupPricingAt_UsesEffectiveDate(t *testing.T) {
+	model := "test-model-windowed"
+	orig, had := defaultPricingHistory[model]
+	if had {
+		defer func() { defaultPricingHistory[model] = orig }()
+	} else {
+		defer delete(defaultPricingHistory, model)
+	}
+
+	defaultPricingHistory[model] = []modelPricingVersion{
+		{
+			EffectiveFrom: mustDate(t, "2025-01-01"),
+			Pricing:       ModelPricing{InputPerMTok: 1.0},
+		},
+		{
+			EffectiveFrom: mustDate(t, "2025-07-01"),
+			Pricing:       ModelPricing{InputPerMTok: 2.0},
+		},
+	}
+
+	aprPrice, ok := LookupPricingAt(model, mustDate(t, "2025-04-15"))
+	if !ok {
+		t.Fatal("LookupPricingAt returned !ok for historical model")
+	}
+	if aprPrice.InputPerMTok != 1.0 {
+		t.Fatalf("April price InputPerMTok = %.2f, want 1.0", aprPrice.InputPerMTok)
+	}
+
+	augPrice, ok := LookupPricingAt(model, mustDate(t, "2025-08-15"))
+	if !ok {
+		t.Fatal("LookupPricingAt returned !ok for historical model in later window")
+	}
+	if augPrice.InputPerMTok != 2.0 {
+		t.Fatalf("August price InputPerMTok = %.2f, want 2.0", augPrice.InputPerMTok)
+	}
+}
+
+func TestLookupPricingAt_UsesLatestWhenTimeZero(t *testing.T) {
+	model := "test-model-latest"
+	orig, had := defaultPricingHistory[model]
+	if had {
+		defer func() { defaultPricingHistory[model] = orig }()
+	} else {
+		defer delete(defaultPricingHistory, model)
+	}
+
+	defaultPricingHistory[model] = []modelPricingVersion{
+		{
+			EffectiveFrom: mustDate(t, "2025-01-01"),
+			Pricing:       ModelPricing{InputPerMTok: 1.0},
+		},
+		{
+			EffectiveFrom: mustDate(t, "2025-09-01"),
+			Pricing:       ModelPricing{InputPerMTok: 3.0},
+		},
+	}
+
+	price, ok := LookupPricingAt(model, time.Time{})
+	if !ok {
+		t.Fatal("LookupPricingAt returned !ok for model with pricing history")
+	}
+	if price.InputPerMTok != 3.0 {
+		t.Fatalf("zero-time lookup InputPerMTok = %.2f, want 3.0", price.InputPerMTok)
+	}
+}