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/vivado_env.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+"""Resolve the machine-local Vivado install path.
+
+The install path differs per machine (C:\\Xilinx here, D:\\Xilinx elsewhere), so it
+is never hardcoded in a skill. It lives in ~/skills_env.md, which stays out of git.
+Precedence: explicit --vivado-dir > OS env var > ~/skills_env.md.
+"""
+import os
+import re
+import sys
+
+
+def env_lookup(key, default=None):
+    """Read KEY from the OS environment, else from a `KEY = value` line in
+    ~/skills_env.md. Trailing `# comments` are stripped."""
+    v = os.environ.get(key)
+    if v:
+        return v
+    path = os.path.join(os.path.expanduser("~"), "skills_env.md")
+    try:
+        with open(path, encoding="utf-8") as f:
+            for line in f:
+                m = re.match(rf"\s*{re.escape(key)}\s*=\s*(.+?)\s*(?:#.*)?$", line)
+                if m:
+                    val = m.group(1).strip()
+                    # Placeholders like `<未安装>` mean "not configured on this machine".
+                    if val and not val.startswith("<"):
+                        return val
+    except OSError:
+        pass
+    return default
+
+
+def resolve_vivado_dir(cli_value=None):
+    """Vivado install ROOT (the dir containing bin/vivado.bat)."""
+    if cli_value:
+        return cli_value
+    root = env_lookup("VIVADO_ROOT")
+    if root:
+        return root
+    exe = env_lookup("VIVADO_PATH")
+    if exe:
+        # .../<root>/bin/vivado.bat -> <root>
+        return os.path.dirname(os.path.dirname(exe))
+    sys.exit(
+        "Vivado path not configured. Add `VIVADO_ROOT = <install root>` to "
+        "~/skills_env.md, set it as an env var, or pass --vivado-dir."
+    )
+
+
+def resolve_vivado_exe(cli_value=None):
+    """Full path to bin/vivado.bat."""
+    if cli_value:
+        return cli_value
+    exe = env_lookup("VIVADO_PATH")
+    if exe:
+        return exe
+    return os.path.join(resolve_vivado_dir(), "bin", "vivado.bat")