fix(budget): add cost fallback for all zero-pricing providers

Pi-ai ships $0 pricing for github-copilot (25 models), google-gemini-cli
(6 models), zai (13 models), and kimi-coding (2 models). This made cost
tracking produce $0 for every run through these providers, rendering
budget enforcement useless.

Added resolveModelCostFallback() in model-resolver.ts with two strategies:
1. Cross-provider lookup: subscription providers (github-copilot,
   google-gemini-cli) use the same models as their API counterparts.
   Look up the model in anthropic/openai/google/xai catalogs using
   normalized ID matching (dots→hyphens) to find real pricing.
2. Per-provider fallback: API-key providers with no cross-provider
   equivalent (zai, kimi-coding) get a static per-provider cost.

CostWriter now calls resolveModelCostFallback after resolveModel, so
all consumers get patched pricing without touching the pi-ai library.

src/main/agent-runtime/cost-writer.ts

@@ -1,7 +1,7 @@
-import { calculateCost, type Usage } from "@mariozechner/pi-ai";
+import { calculateCost, type Usage, type Api, type Model } from "@mariozechner/pi-ai";
 import type { AppDatabase } from "@main/database";
 import { getConnectorDefinition } from "@main/connectors";
-import { resolveModel } from "@main/model-resolver";
+import { resolveModel, resolveModelCostFallback } from "@main/model-resolver";
 
 export interface CostWriterDeps {
   db: AppDatabase;
@@ -17,19 +17,10 @@ interface ObserveInput {
   usage: Usage;
 }
 
-/**
- * Writes a `cost_entries` row for each pi `message_end`. finishRun reads
- * SUM(cost_entries.amount_usd) for the run-level aggregate, so this is
- * the single source of truth for cost attribution.
- */
 export class CostWriter {
   constructor(private readonly deps: CostWriterDeps) {}
 
   observeAssistantMessage(input: ObserveInput): void {
-    // Fail closed on cost calculation — budget policies read `cost_entries`
-    // to enforce spending gates; silently emitting an unattributed row
-    // would let an agent run unbounded. If pricing cannot be resolved,
-    // throw so the caller logs and the missing cost is visible.
     const definition = getConnectorDefinition(input.connectorId);
     if (!definition.provider) {
       throw new Error(`[cost-writer] connector ${input.connectorId} has no provider mapping`);
@@ -38,7 +29,8 @@ export class CostWriter {
     if (!model) {
       throw new Error(`[cost-writer] model ${input.modelId} not found for provider ${definition.provider}`);
     }
-    const cost = calculateCost(model, input.usage);
+    const patchedModel = resolveModelCostFallback(model, definition.provider);
+    const cost = calculateCost(patchedModel, input.usage);
     if (!Number.isFinite(cost.total)) {
       throw new Error(
         `[cost-writer] cost calculation returned non-finite total for ${input.connectorId}/${input.modelId}`,