fix: shutdown ACP backend on session kill to reap CC child processes (#3184)

Call acpBackend.shutdownSession() before sessions.killSession() in DELETE /v1/sessions/:id handler. Prevents orphaned Claude Code processes (~250MB RSS each) that accumulate and cause OOM kills.\n\nFixes #3184

Author: OneStepAt4time

src/routes/sessions.ts

@@ -291,6 +291,15 @@ export function registerSessionRoutes(app: FastifyInstance, ctx: RouteContext):
         continue;
       }
       try {
+        // #3184: Shutdown ACP backend (CC child process) before marking session killed.
+        // Without this, the CC process becomes an orphan consuming ~250MB RSS.
+        if (acpBackend && ctx.config.acpEnabled) {
+          await acpBackend.shutdownSession({
+            sessionId: id,
+            tenantId: req.tenantId ?? SYSTEM_TENANT,
+            ownerKeyId: req.authKeyId ?? 'master',
+          }).catch(() => {}); // Best-effort: session may not have ACP runtime
+        }
         await sessions.killSession(id);
         eventBus.emitEnded(id, 'killed');
         void channels.sessionEnded(makePayload(sessions, 'session.ended', id, 'killed'));