fix(parser): 修复 bufio.Scanner token too long 错误 (#64)

## 问题
- 执行 rg 等命令时，如果匹配到 minified 文件，单行输出可能超过 10MB
- 旧实现使用 bufio.Scanner，遇到超长行会报错并中止整个解析
- 导致后续的 agent_message 无法读取，任务失败

## 修复
1. **parser.go**:
   - 移除 bufio.Scanner，改用 bufio.Reader + readLineWithLimit
   - 超长行（>10MB）会被跳过但继续处理后续事件
   - 添加 codexHeader 轻量级解析，只在 agent_message 时完整解析

2. **utils.go**:
   - 修复 logWriter 内存膨胀问题
   - 添加 writeLimited 方法限制缓冲区大小

3. **测试**:
   - parser_token_too_long_test.go: 验证超长行处理
   - log_writer_limit_test.go: 验证日志缓冲限制

## 测试结果
- ✅ TestParseJSONStream_SkipsOverlongLineAndContinues
- ✅ TestLogWriterWriteLimitsBuffer
- ✅ 完整测试套件通过

Fixes #64

Generated with swe-agent-bot

Co-Authored-By: swe-agent-bot <agent@swe-agent.ai>

codeagent-wrapper/parser_token_too_long_test.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseJSONStream_SkipsOverlongLineAndContinues(t *testing.T) {
+	// Exceed the 10MB bufio.Scanner limit in parseJSONStreamInternal.
+	tooLong := strings.Repeat("a", 11*1024*1024)
+
+	input := strings.Join([]string{
+		`{"type":"item.completed","item":{"type":"other_type","text":"` + tooLong + `"}}`,
+		`{"type":"thread.started","thread_id":"t-1"}`,
+		`{"type":"item.completed","item":{"type":"agent_message","text":"ok"}}`,
+	}, "\n")
+
+	var warns []string
+	warnFn := func(msg string) { warns = append(warns, msg) }
+
+	gotMessage, gotThreadID := parseJSONStreamInternal(strings.NewReader(input), warnFn, nil, nil)
+	if gotMessage != "ok" {
+		t.Fatalf("message=%q, want %q (warns=%v)", gotMessage, "ok", warns)
+	}
+	if gotThreadID != "t-1" {
+		t.Fatalf("threadID=%q, want %q (warns=%v)", gotThreadID, "t-1", warns)
+	}
+	if len(warns) == 0 || !strings.Contains(warns[0], "Skipped overlong JSON line") {
+		t.Fatalf("expected warning about overlong JSON line, got %v", warns)
+	}
+}