fix: make TAIL_READ_BYTES internal constant instead of exported

TAIL_READ_BYTES is only used internally within readLastJsonlEntry()
and is not exported from the package's index.ts. Removed the export
keyword to make it clear it's an internal implementation detail.

Addresses bugbot comment about unused export.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/core/src/utils.ts

@@ -36,7 +36,7 @@ export function validateUrl(url: string, label: string): void {
  * Read only the last 4KB of JSONL files to find the last entry.
  * Avoids loading entire (potentially large) files into memory.
  */
-export const TAIL_READ_BYTES = 4096;
+const TAIL_READ_BYTES = 4096;
 
 /**
  * Read the last entry from a JSONL file efficiently.