vivado: resolve install path from ~/skills_env.md instead of hardcoding D:\Xilinx

The Vivado install root differs per machine (C:\Xilinx here, D:\Xilinx on the
box these skills were written on), so every hardcoded path was already wrong
on this machine.

- new fpga_flash/scripts/vivado_env.py: resolves --vivado-dir > env var >
  VIVADO_ROOT (falling back to VIVADO_PATH) from ~/skills_env.md
- gen_mcs / program_flash / check_flash_csv: --vivado-dir now optional
- fpga_update/gen_update_mcs.py: same precedence for --vivado
- fpga_flash / smod_atsc_dev / vivado-prj-create SKILL.md: document the
  lookup instead of naming a drive letter
- skills_env template: VIVADO_ROOT/VIVADO_PATH are now blanks to fill in,
  so a new machine can't inherit a wrong default

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools-sync/references/skills_env_template.md

@@ -27,7 +27,8 @@ CONDA_LANGGRAPH_PYTHON  = C:\Users\<user>\.conda\envs\langgraph\python.exe
 ## EDA / 硬件工具
 
 ```
-VIVADO_PATH   = D:\Xilinx\Vivado\2018.1\bin\vivado.bat
+VIVADO_ROOT   = <Vivado 安装根目录，如 C:\Xilinx\Vivado\2018.1 —— 必须实测，不同机器盘符不同>
+VIVADO_PATH   = <上一行 + \bin\vivado.bat>
 KICAD_PYTHONPATH = C:\Program Files\KiCad\10.0\bin\Lib\site-packages
 MATLAB_MCP_EXE = C:\Users\<user>\.matlab\agentic-toolkits\bin\matlab-mcp-server.exe
 ```