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/agents/pi-embedded-subscribe.handlers.lifecycle.ts

@@ -21,6 +21,7 @@ export function handleAgentStart(ctx: EmbeddedPiSubscribeContext) {
 
 export function handleAutoCompactionStart(ctx: EmbeddedPiSubscribeContext) {
   ctx.state.compactionInFlight = true;
+  ctx.incrementCompactionCount();
   ctx.ensureCompactionPromise();
   ctx.log.debug(`embedded run compaction start: runId=${ctx.params.runId}`);
   emitAgentEvent({