fix(agency): UsageTotals carries + accumulates cache-token fields

Final unswept accumulator surface. UsageTotals (the agency() API's
internal helper type) matched the same pattern as the pre-fix
generateObject / streamText / strategies / UsageLedger / ITokenUsage
types — promptTokens + completionTokens + totalTokens + costUSD but no
cacheReadTokens / cacheCreationTokens. normalizeUsage dropped cache
fields from the source usage; addUsageTotals never summed them even
when callers routed cache-token-reporting usage objects through.

With this fix, every AgentOS accumulator I could find in src/ forwards
Anthropic prompt-cache counters end-to-end. Consumers using the public
agency() API now see cache-aware totals with the same undefined-vs-zero
convention as the rest of the surface.

No behavior change for non-cache paths. Typecheck clean.

Author: jddunn

src/api/agency.ts

@@ -977,6 +977,10 @@ type UsageTotals = {
   completionTokens: number;
   totalTokens: number;
   costUSD?: number;
+  /** Anthropic `cache_read_input_tokens`. Undefined when no source reported. */
+  cacheReadTokens?: number;
+  /** Anthropic `cache_creation_input_tokens`. Same undefined convention. */
+  cacheCreationTokens?: number;
 };
 
 function emptyUsageTotals(): UsageTotals {
@@ -990,6 +994,10 @@ function normalizeUsage(raw: unknown): UsageTotals {
     completionTokens: usage.completionTokens ?? 0,
     totalTokens: usage.totalTokens ?? 0,
     costUSD: usage.costUSD,
+    // Preserve cache-token fields from the source. Undefined stays
+    // undefined so consumers distinguish "not reported" from "zero".
+    cacheReadTokens: usage.cacheReadTokens,
+    cacheCreationTokens: usage.cacheCreationTokens,
   };
 }
 
@@ -1000,6 +1008,12 @@ function addUsageTotals(target: UsageTotals, usage: UsageTotals): void {
   if (typeof usage.costUSD === 'number') {
     target.costUSD = (target.costUSD ?? 0) + usage.costUSD;
   }
+  if (typeof usage.cacheReadTokens === 'number') {
+    target.cacheReadTokens = (target.cacheReadTokens ?? 0) + usage.cacheReadTokens;
+  }
+  if (typeof usage.cacheCreationTokens === 'number') {
+    target.cacheCreationTokens = (target.cacheCreationTokens ?? 0) + usage.cacheCreationTokens;
+  }
 }
 
 function getSessionUsage(