Remove dead code: scan_pid_files, backend_label, entries_mut, post_json

All confirmed unused anywhere in src/ or channels/.

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

src/agent/mod.rs

@@ -1008,8 +1008,6 @@ impl Agent {
         self.client.clone()
     }
 
-    pub fn entries_mut(&mut self) -> &mut Vec<ConversationEntry> {
-        &mut self.context.entries
-    }
+
 
 }