fix(memory): always reconcile line count in TruncateHistory

A crash between the JSONL append and the meta update in addMsg can
leave meta.Count stale (e.g. file has 101 lines but meta says 100).
The previous code only reconciled when Count==0, so a nonzero stale
count was silently trusted, causing keepLast/skip to be calculated
against the wrong total.

Now TruncateHistory always counts the actual lines on disk. This is
cheap (scan without unmarshal) and TruncateHistory is not a hot path.

pkg/memory/jsonl.go

@@ -328,15 +328,16 @@ func (s *JSONLStore) TruncateHistory(
 		return err
 	}
 
-	// If the meta count might be stale (e.g. after a crash during
-	// addMsg), reconcile with the actual line count on disk.
-	if meta.Count == 0 {
-		n, countErr := countLines(s.jsonlPath(sessionKey))
-		if countErr != nil {
-			return countErr
-		}
-		meta.Count = n
+	// Always reconcile meta.Count with the actual line count on disk.
+	// A crash between the JSONL append and the meta update in addMsg
+	// leaves meta.Count stale (e.g. file has 101 lines but meta says
+	// 100). Counting lines is cheap — no unmarshal, just a scan — and
+	// TruncateHistory is not a hot path, so always re-count.
+	n, countErr := countLines(s.jsonlPath(sessionKey))
+	if countErr != nil {
+		return countErr
 	}
+	meta.Count = n
 
 	if keepLast <= 0 {
 		meta.Skip = meta.Count

pkg/memory/jsonl_test.go

@@ -544,6 +544,57 @@ func TestCompact_ThenAppend(t *testing.T) {
 	}
 }
 
+func TestTruncateHistory_StaleMetaCount(t *testing.T) {
+	// Simulates a crash between JSONL append and meta update in addMsg:
+	// file has N+1 lines but meta.Count is still N. TruncateHistory must
+	// reconcile with the real line count so that keepLast is accurate.
+	store := newTestStore(t)
+	ctx := context.Background()
+
+	// Write 10 messages normally (meta.Count = 10).
+	for i := 0; i < 10; i++ {
+		err := store.AddMessage(ctx, "stale", "user", string(rune('a'+i)))
+		if err != nil {
+			t.Fatalf("AddMessage: %v", err)
+		}
+	}
+
+	// Simulate crash: append a line to JSONL but do NOT update meta.
+	// This leaves meta.Count = 10 while the file has 11 lines.
+	jsonlPath := store.jsonlPath("stale")
+	f, err := os.OpenFile(jsonlPath, os.O_WRONLY|os.O_APPEND, 0o644)
+	if err != nil {
+		t.Fatalf("open for append: %v", err)
+	}
+	_, err = f.WriteString(`{"role":"user","content":"orphan"}` + "\n")
+	if err != nil {
+		t.Fatalf("write orphan: %v", err)
+	}
+	f.Close()
+
+	// TruncateHistory(keepLast=4) should keep the last 4 of 11 lines,
+	// not the last 4 of 10.
+	err = store.TruncateHistory(ctx, "stale", 4)
+	if err != nil {
+		t.Fatalf("TruncateHistory: %v", err)
+	}
+
+	history, err := store.GetHistory(ctx, "stale")
+	if err != nil {
+		t.Fatalf("GetHistory: %v", err)
+	}
+	if len(history) != 4 {
+		t.Fatalf("expected 4, got %d", len(history))
+	}
+	// Last 4 of [a,b,c,d,e,f,g,h,i,j,orphan] = [h,i,j,orphan]
+	if history[0].Content != "h" {
+		t.Errorf("first kept = %q, want 'h'", history[0].Content)
+	}
+	if history[3].Content != "orphan" {
+		t.Errorf("last kept = %q, want 'orphan'", history[3].Content)
+	}
+}
+
 func TestCrashRecovery_PartialLine(t *testing.T) {
 	store := newTestStore(t)
 	ctx := context.Background()