feat(utils): support user config journal default
Extend get_journal_info() resolution order to: env -> user config (~/.config/solstone/config.toml) -> source-tree -> ~/Documents/journal default.
New module think/user_config.py exposes default_journal(), config_path(), read_user_config(), and write_user_config() for a future first-run setup tool to persist the journal location without touching the wrapper or env vars.
get_journal_info() no longer raises; SolstoneNotConfigured now only fires when get_journal()'s mkdir fails. sol config show learns sensible labels for the new config and default sources. docs/environment.md updated to reflect the four-step order.