refactor: fine tune config connection manner and errmsgs

[tyranitar-mega]

fixes #37

Main changes:

1. config set-connection will now create a config file if missed. get-connections will show No connections configured. use-connection and delete-connection will show Connection xxx not found. Users will not be aware of the existence of the config file unless checking the logs (no config file found in [/root/.config/scopeql/config.toml, /root/.scopeql/config.toml]; treating as empty connections)
2. When the config file is broken, there will be a line that indicates which file was read from the log file like {"timestamp":"2026-05-24T20:48:31.472035+08:00","level":"INFO","target":"scopeql::config","file":"scopeql/src/config.rs","line":70,"message":"loaded config from /root/.config/scopeql/config.toml"}

Example:

$ ./target/debug/scopeql config use-connection cloud
Failed to switch connection: failed to deserialize config on /root/.config/scopeql/config.toml: missing field `auth`
in `connections.cloud`

$ ./target/debug/scopeql config delete-connection cloud
Failed to delete connection: failed to deserialize config on /root/.config/scopeql/config.toml: missing field `auth`
in `connections.cloud`

$ ./target/debug/scopeql config get-connections
Failed to load config: failed to deserialize config on /root/.config/scopeql/config.toml: missing field `auth`
in `connections.cloud`