feat(ui): Expose history truncation controls in the Discussion panel

gui.py

@@ -62,6 +62,15 @@ def get_total_token_usage() -> dict:
                 usage[k] += u.get(k, 0) or 0
     return usage
 
+def truncate_entries(entries: list[dict], max_pairs: int) -> list[dict]:
+    """Truncates history to the last N pairs of User/AI messages."""
+    if max_pairs <= 0:
+        return []
+    target_count = max_pairs * 2
+    if len(entries) <= target_count:
+        return entries
+    return entries[-target_count:]
+
 
 # ------------------------------------------------------------------ comms rendering helpers
 
@@ -1344,6 +1353,11 @@ class App:
         self.disc_entries.clear()
         self._rebuild_disc_list()
 
+    def cb_disc_truncate(self):
+        self.disc_entries = truncate_entries(self.disc_entries, 2)
+        self._rebuild_disc_list()
+        self._update_status("history truncated")
+
     def cb_disc_collapse_all(self):
         for i, entry in enumerate(self.disc_entries):
             tag = f"disc_content_{i}"
@@ -1761,6 +1775,7 @@ class App:
                 dpg.add_button(label="+ Entry", callback=self.cb_disc_append_entry)
                 dpg.add_button(label="-All", callback=self.cb_disc_collapse_all)
                 dpg.add_button(label="+All", callback=self.cb_disc_expand_all)
+                dpg.add_button(label="Truncate", callback=self.cb_disc_truncate)
                 dpg.add_button(label="Clear All", callback=self.cb_disc_clear)
                 dpg.add_button(label="Save", callback=self.cb_disc_save)
             dpg.add_checkbox(