fix: context overflow compaction and subagent announce improvements (#11664) (thanks @tyler6204)

* initial commit

* feat: implement deriveSessionTotalTokens function and update usage tests

* Added deriveSessionTotalTokens function to calculate total tokens based on usage and context tokens.
* Updated usage tests to include cases for derived session total tokens.
* Refactored session usage calculations in multiple files to utilize the new function for improved accuracy.

* fix: restore overflow truncation fallback + changelog/test hardening (#11551) (thanks @tyler6204)

src/cron/isolated-agent/run.ts

@@ -33,7 +33,7 @@ import { buildWorkspaceSkillSnapshot } from "../../agents/skills.js";
 import { getSkillsSnapshotVersion } from "../../agents/skills/refresh.js";
 import { runSubagentAnnounceFlow } from "../../agents/subagent-announce.js";
 import { resolveAgentTimeoutMs } from "../../agents/timeout.js";
-import { hasNonzeroUsage } from "../../agents/usage.js";
+import { deriveSessionTotalTokens, hasNonzeroUsage } from "../../agents/usage.js";
 import { ensureAgentWorkspace } from "../../agents/workspace.js";
 import {
   normalizeThinkLevel,
@@ -454,11 +454,13 @@ export async function runCronIsolatedAgentTurn(params: {
     if (hasNonzeroUsage(usage)) {
       const input = usage.input ?? 0;
       const output = usage.output ?? 0;
-      const promptTokens = input + (usage.cacheRead ?? 0) + (usage.cacheWrite ?? 0);
       cronSession.sessionEntry.inputTokens = input;
       cronSession.sessionEntry.outputTokens = output;
       cronSession.sessionEntry.totalTokens =
-        promptTokens > 0 ? promptTokens : (usage.total ?? input);
+        deriveSessionTotalTokens({
+          usage,
+          contextTokens,
+        }) ?? input;
     }
     await persistSessionEntry();
   }