agent: drop error return from Session.update

The function only appends to a slice and can never fail. Removing the
dead error return eliminates the nolint:errcheck suppression on the
interrupted-stream path in loop.go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -110,7 +110,7 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 					IsError:    true,
 				})
 			}
-			state.update(msg, results) //nolint:errcheck
+			state.update(msg, results)
 			return fmt.Errorf("step %d: stream interrupted: %w", step, ctx.Err())
 		}
 
@@ -235,9 +235,7 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 			}
 		}
 
-		if err := state.update(msg, results); err != nil {
-			return fmt.Errorf("step %d update: %w", step, err)
-		}
+		state.update(msg, results)
 
 		if interrupted {
 			return ctx.Err()

pkg/agent/session.go

@@ -94,7 +94,7 @@ func (s *Session) addUsage(u backend.Usage, estimated bool) {
 	}
 }
 
-func (s *Session) update(assistant backend.Message, results []toolResult) error {
+func (s *Session) update(assistant backend.Message, results []toolResult) {
 	s.messages = append(s.messages, assistant)
 	for _, r := range results {
 		s.messages = append(s.messages, backend.Message{
@@ -103,7 +103,6 @@ func (s *Session) update(assistant backend.Message, results []toolResult) error
 			ToolCallID: r.ToolCallID,
 		})
 	}
-	return nil
 }
 
 // PreCompactionSnapshot returns a copy of the current messages for persistence

pkg/agent/state.go

@@ -19,6 +19,6 @@ type state interface {
 
 	// Update appends the assistant's reply and any tool results to the history.
 	// Called once per loop iteration after Act completes.
-	update(assistant backend.Message, results []toolResult) error
+	update(assistant backend.Message, results []toolResult)
 
 }