truncation: avoid mid-rune cut with ToValidUTF8

pkg/agent/loop.go

@@ -345,7 +345,7 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 			}
 			if !isErr && cfg.ToolResultMaxBytes > 0 && len(result) > cfg.ToolResultMaxBytes {
 				orig := len(result)
-				result = result[:cfg.ToolResultMaxBytes]
+				result = strings.ToValidUTF8(result[:cfg.ToolResultMaxBytes], "")
 				result += fmt.Sprintf("\n\n[HARD LIMIT: %s output truncated — %d of %d bytes shown. This is a safety ceiling, not a semantic boundary.]",
 					tc.Name, cfg.ToolResultMaxBytes, orig)
 			}