fix(omp): avoid unsupported calculateCost import

Fix OMP installation by removing the unsupported calculateCost runtime import from pi-ai.

src/cost.ts

@@ -0,0 +1,19 @@
+/**
+ * Local cost calculation for Command Code usage.
+ *
+ * Mirrors pi-ai's `calculateCost` arithmetic exactly. The provider ships its
+ * own copy because Oh My Pi's legacy pi-ai shim does not export
+ * `calculateCost`, which broke extension installation there (issue #24).
+ * `tests/test-cost.ts` locks this implementation to the pi-ai original.
+ */
+
+import type { ModelLike, Usage } from "./types.ts"
+
+export function calculateCommandCodeCost(model: ModelLike, usage: Usage): void {
+  usage.cost.input = (model.cost.input / 1_000_000) * usage.input
+  usage.cost.output = (model.cost.output / 1_000_000) * usage.output
+  usage.cost.cacheRead = (model.cost.cacheRead / 1_000_000) * usage.cacheRead
+  usage.cost.cacheWrite = (model.cost.cacheWrite / 1_000_000) * usage.cacheWrite
+  usage.cost.total =
+    usage.cost.input + usage.cost.output + usage.cost.cacheRead + usage.cost.cacheWrite
+}

src/types.ts

@@ -50,11 +50,19 @@ export interface AssistantMessageLike {
   timestamp: number
 }
 
+export interface ModelCost {
+  input: number
+  output: number
+  cacheRead: number
+  cacheWrite: number
+}
+
 export interface ModelLike {
   id: string
   api: unknown
   provider: string
   maxTokens: number
+  cost: ModelCost
 }
 
 export interface MessageLike {