build: 删除 MDK-ARM/download.bat，烧录收敛到 stlink-flash skill

- test/run_all_suites.py 是唯一执行它的地方：DOWNLOAD_BAT 改为 STLINK_FLASH
  （.dsh/skills/stlink-flash/scripts/stlink_flash.py）；直烧改
  [py, STLINK_FLASH, "flash"]（原先把 .bat 包给命令行解释器执行）；
  --flash-cmd 改由同一常量拼出，不再硬编码 .bat 路径
- 机制变化（须知晓）：原 download.bat 走 UV4 -f（CMSIS-DAP），现走
  ST-LINK_CLI -P <hex> -Rst -Run —— 与 .serialtest/config.json 的
  flash_cmd、AGENTS.md 烧录铁律、回读 -CmpFile 要求同源；6.5s/1MB
  也快于 UV4 路径
- AGENTS.md / 环境与部署 3 篇 / ST-LINK_CLI 复位说明（原文写"正式流程仍以
  download.bat 为准"，已反转）/ 自动化测试流程 / 串口助手设计说明 /
  链路延迟 / m2_data / README / MDK-ARM/README：引用改指 skill
- .gitignore：download_log.txt 条目保留并注明 legacy（他机残留物不变
  untracked）；build 日志注释同步
- 保留 MDK-ARM/reset.bat：其 hard/halt/run 子命令是 skill 没有的
- 刻意未改：docs/问题记录/项目审查与优化建议报告.md 的 P2-15（史实）
- 验证：run_all_suites.py --dry-run 解析出 skill 路径；doc-links 123 通过

MDK-ARM/README.md

@@ -10,8 +10,7 @@
 | `STM32F407-Demo.uvprojx` | **Keil 工程文件**（源文件清单、编译选项、目标配置） |
 | `STM32F407-Demo.uvoptx` | Keil 用户选项（调试器、断点等） |
 | `startup_stm32f407xx.s` | 启动文件（**`Heap_Size EQU 0x200`（512B）在此限定**） |
-| `download.bat` | 烧录（UV4 路径） |
-| `reset.bat` | **复位/烧录**（`ST-LINK_CLI`，见下） |
+| `reset.bat` | **复位/烧录/回读**（`ST-LINK_CLI` 封装，见下） |
 
 ## 构建与烧录
 

MDK-ARM/download.bat

@@ -1,45 +0,0 @@
-@echo off
-setlocal
-
-set PROJ=%~dp0STM32F407-Demo.uvprojx
-set LOG=%~dp0download_log.txt
-set TARGET=STM32F407-Demo
-
-:: UV4 location: env var DTU_KEIL_UV4 = full path of UV4.exe (same contract
-:: as keil_build.py (@build); required, no machine-default fallback).
-if defined DTU_KEIL_UV4 goto uv4_set
-echo DOWNLOAD_FAIL: environment variable DTU_KEIL_UV4 is not set
-echo Set it to the full path of UV4.exe, e.g.:
-echo   [Environment]::SetEnvironmentVariable('DTU_KEIL_UV4', 'D:\Tools\Keil\UV4\UV4.exe', 'User')
-exit /b 2
-
-:uv4_set
-if exist "%DTU_KEIL_UV4%" goto uv4_ok
-echo ENV_ERROR: DTU_KEIL_UV4 is set but does not exist: "%DTU_KEIL_UV4%"
-echo Fix the variable value. No fallback to another UV4 install is attempted.
-exit /b 2
-
-:uv4_ok
-set UV4=%DTU_KEIL_UV4%
-
-:: Remove stale log first: UV4 -o writes the log only when a download actually
-:: runs. A leftover log would otherwise be misreported as this download's
-:: result (same guard as keil_build.py).
-if exist "%LOG%" del "%LOG%"
-
-"%UV4%" -f "%PROJ%" -t%TARGET% -j0 -o "%LOG%"
-
-if not exist "%LOG%" (
-    echo DOWNLOAD_FAIL: no download log produced - UV4 did not run or hung
-    exit /b 1
-)
-
-findstr /C:"Application running" "%LOG%" >nul
-if %ERRORLEVEL% equ 0 (
-    echo DOWNLOAD_OK: flashed and running
-    exit /b 0
-)
-
-echo DOWNLOAD_FAIL: no application-running marker in log
-type "%LOG%"
-exit /b 1