fix(subagent): cap truncated announce text to maxAnnounceChars

- Reserve space for truncation marker before slicing findings
- Prevents final result from exceeding configured character limit
- Addresses Codex review feedback on PR #46863

Before: findings.slice(max) + marker (exceeds max by marker length)
After: findings.slice(max - marker.length) + marker (respects max)

src/agents/subagent-announce.ts

@@ -1398,12 +1398,12 @@ export async function runSubagentAnnounceFlow(params: {
     });
 
     // Apply maxAnnounceChars truncation if specified
+    const TRUNCATION_MARKER = "\n\n[truncated — full output in transcript]";
     const truncatedFindings =
       typeof params.maxAnnounceChars === "number" &&
       params.maxAnnounceChars >= 1 &&
       findings.length > params.maxAnnounceChars
-        ? findings.slice(0, params.maxAnnounceChars) +
-          "\n\n[truncated — full output in transcript]"
+        ? findings.slice(0, params.maxAnnounceChars - TRUNCATION_MARKER.length) + TRUNCATION_MARKER
         : findings;
 
     const internalEvents: AgentInternalEvent[] = [