fix(cli): stop flag defaults from clobbering config-file values

[fiskhest]

PR: fix(cli): stop flag defaults from clobbering config-file values

Base branch: mainline
Branch: fix/cli-flag-config-overrides
Commit: e8930a8 (patch: /tmp/opencode/skim-fix.patch)

Summary

When a config file is loaded with skim generate -c <file>, three CLI flags
unconditionally overwrite the corresponding config-file values even when the
flag is not passed on the command line:

• -F/--use-system-fonts -> clobbers output.style.use_system_fonts
• -N/--no-special-keys -> clobbers output.style.legend_tables.macros/tap_dances.show
• -Y/--no-symbols -> clobbers output.style.legend_tables.symbols.show

Because the flags defaulted to False/True (not None), "flag absent" was
indistinguishable from "user explicitly passed the default", so
_get_config() had no way to know it should keep the config value.

Repro

# skimconfig.yaml
output:
  style:
    use_system_fonts: true

skim generate -c skimconfig.yaml -k keyboard.kbi -o layout/ --layer 0-3
grep -c @font-face layout/keymap-layer-1.svg   # -> 5 (fonts embedded anyway)

Expected: 0 (config honored without -F). Same for
legend_tables.*.show: false vs -N/-Y.

Fix

Make the three flags tri-state (None = not given) and only apply them in
_get_config() when not None.

• src/skim/cli.py — --use-system-fonts, -N/--no-special-keys,
  -Y/--no-symbols click options now default=None; generate() signature
  and the generate_keymap() call site pass None through for absent flags.
• src/skim/data/cli.py — OutputFiles.use_system_fonts defaults to None.
• src/skim/application/keymap_generator.py — _get_config() only applies
  use_system_fonts, show_special_keys_legend, and show_symbol_legend
  when the argument is not None; generate_keymap() widens the two legend
  params to bool | None.
• src/skim/application/exporter/__init__.py — save_drawings() now takes
  the resolved use_system_fonts value (previously it re-read the raw CLI
  value, so cairo/PNG export would ignore the config).

Tests

• tests/unit/application/test_keymap_generator.py — precedence matrix for
  _get_config(): config value preserved when flag absent; explicit
  True/False still wins.
• tests/unit/test_cli.py — absent flags propagate as None; -F, -N,
  -Y propagate their values.
• tests/unit/data/test_cli.py — OutputFiles.use_system_fonts default is
  now None.

Verification

• uv run pytest tests/unit — 1410 passed, 1 skipped (81.61% coverage, meets 80%)
• uv run ruff check src tests — clean
• uv run ruff format --check src tests — clean
• uv run basedpyright src — 0 errors
• Manual:
  • config use_system_fonts: true, no flag -> @font-face count 0
  • -F flag -> @font-face count 0
  • no config, no flag -> @font-face count 5 (defaults unchanged)