Fix: mode from config file is ignored (fixes #97)

[galopyz]

Fixes #97

The CLI default for mode was always 'default', so get_opts never read the mode value from shell_sage.conf. Use None as the CLI default so configured values like mode = sassy can take effect.

• main(): mode: str = 'default' → mode: str = None
• main(): pass mode=mode to get_opts(...)
• main(): use opts.mode for validation, the get_sage(...) call, and the Log insert

Verified: with mode = sassy in ~/.config/shell_sage/shell_sage.conf, ssage now responds in sassy mode; --mode default still overrides the config.

I did not add a regression test but I'd be happy to add one if that would help.