make skillgen's toml import work on Python 3.10

gen.py imported tomllib unconditionally, but that is 3.11+ stdlib only and
graphify supports (and CI tests) 3.10 - so importing tools.skillgen.gen, and
thus collecting test_skillgen.py, crashed the 3.10 test job with
ModuleNotFoundError. Fall back to the tomli backport on <3.11 and add tomli to
the dev group under that marker. Verified the fallback loads and parses with
tomllib forced absent.

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

pyproject.toml

@@ -89,6 +89,7 @@ dev = [
     "safety>=3.7.0",
     "setuptools>=82.0.1",
     "wheel>=0.47.0",
+    "tomli>=2.0 ; python_version < '3.11'",
 ]
 
 [tool.uv]

tools/skillgen/gen.py

@@ -27,7 +27,10 @@ import argparse
 import re
 import subprocess
 import sys
-import tomllib
+try:
+    import tomllib  # Python 3.11+ stdlib
+except ModuleNotFoundError:  # Python 3.10 - graphify supports >=3.10
+    import tomli as tomllib  # type: ignore[no-redef]
 from dataclasses import dataclass, field
 from pathlib import Path