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/check_flash_csv.py

@@ -6,7 +6,8 @@ Checks whether 'w25q128bv-spi-x1_x2_x4' is present in Vivado's
 xicom_cfgmem_part_table.csv. If not, appends the correct row.
 
 Usage:
-    python check_flash_csv.py --vivado-dir "D:/Xilinx/Vivado/2018.1"
+    python check_flash_csv.py            # Vivado path from ~/skills_env.md
+    python check_flash_csv.py --vivado-dir "<install root>"   # or override it
 
 Exit codes:
     0 = device already present (no changes made)
@@ -17,6 +18,8 @@ import argparse
 import os
 import sys
 
+from vivado_env import resolve_vivado_dir
+
 DEVICE_NAME = "w25q128bv-spi-x1_x2_x4"
 # Fields: NAME, MEM_DEVICE, MEM_TYPE, MEM_DENSITY, DATA_WIDTH,
 #         MEM_MANUFACTURER, NUM_CFG_FILES, MEM_DEVICE_ALIAS, MEM_FAMILY
@@ -29,13 +32,14 @@ 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",
     )
     args = parser.parse_args()
 
     csv_path = os.path.join(
-        args.vivado_dir, "data", "xicom", "xicom_cfgmem_part_table.csv"
+        resolve_vivado_dir(args.vivado_dir),
+        "data", "xicom", "xicom_cfgmem_part_table.csv"
     )
     csv_path = csv_path.replace("\\", "/")