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/auto-reply/reply/session-usage.ts

@@ -1,5 +1,9 @@
 import { setCliSessionId } from "../../agents/cli-session.js";
-import { hasNonzeroUsage, type NormalizedUsage } from "../../agents/usage.js";
+import {
+  deriveSessionTotalTokens,
+  hasNonzeroUsage,
+  type NormalizedUsage,
+} from "../../agents/usage.js";
 import {
   type SessionSystemPromptReport,
   type SessionEntry,
@@ -32,15 +36,18 @@ export async function persistSessionUsageUpdate(params: {
         update: async (entry) => {
           const input = params.usage?.input ?? 0;
           const output = params.usage?.output ?? 0;
-          const promptTokens =
-            input + (params.usage?.cacheRead ?? 0) + (params.usage?.cacheWrite ?? 0);
+          const resolvedContextTokens = params.contextTokensUsed ?? entry.contextTokens;
           const patch: Partial<SessionEntry> = {
             inputTokens: input,
             outputTokens: output,
-            totalTokens: promptTokens > 0 ? promptTokens : (params.usage?.total ?? input),
+            totalTokens:
+              deriveSessionTotalTokens({
+                usage: params.usage,
+                contextTokens: resolvedContextTokens,
+              }) ?? input,
             modelProvider: params.providerUsed ?? entry.modelProvider,
             model: params.modelUsed ?? entry.model,
-            contextTokens: params.contextTokensUsed ?? entry.contextTokens,
+            contextTokens: resolvedContextTokens,
             systemPromptReport: params.systemPromptReport ?? entry.systemPromptReport,
             updatedAt: Date.now(),
           };