read_tail 64MB — just read the whole log

Images in the jsonl eat most of the byte budget. 64MB covers
any realistic conversation log; compact() trims to fit.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/runner.rs

@@ -986,7 +986,7 @@ impl Agent {
     /// Returns true if the log had content to restore.
     pub fn restore_from_log(&mut self) -> bool {
         let entries = match &self.conversation_log {
-            Some(log) => match log.read_tail(8 * 1024 * 1024) {
+            Some(log) => match log.read_tail(64 * 1024 * 1024) {
                 Ok(entries) if !entries.is_empty() => entries,
                 _ => return false,
             },