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>

fpga_flash/scripts/program_flash.py

@@ -7,7 +7,6 @@ via -tclargs so no temporary script files are created.
 
 Usage:
     python program_flash.py \
-        --vivado-dir  "D:/Xilinx/Vivado/2018.1" \
         --flash-part  "w25q128bv-spi-x1_x2_x4" \
         --bit-file    "E:/project/impl_1/RU_golden_top.bit" \
         --mcs-file    "E:/project/impl_1/RU_golden_smod.mcs" \
@@ -29,6 +28,8 @@ import os
 import subprocess
 import sys
 
+from vivado_env import resolve_vivado_dir
+
 
 def norm(path: str) -> str:
     """Normalize path to forward slashes (Vivado on Windows accepts both)."""
@@ -41,8 +42,8 @@ def main():
     )
     parser.add_argument(
         "--vivado-dir",
-        required=True,
-        help="Vivado install root, e.g. D:/Xilinx/Vivado/2018.1",
+        default=None,
+        help="Vivado install root; defaults to VIVADO_ROOT from ~/skills_env.md",
     )
     parser.add_argument(
         "--flash-part",
@@ -66,7 +67,7 @@ def main():
     )
     args = parser.parse_args()
 
-    vivado_dir = norm(args.vivado_dir)
+    vivado_dir = norm(resolve_vivado_dir(args.vivado_dir))
     bit_file = norm(args.bit_file)
     mcs_file = norm(args.mcs_file)
     ltx_file = norm(args.ltx_file) if args.ltx_file else ""