feat: support autonaming session

[sigoden]

The autonaming session will only be enabled if the following two conditions are met:

1. Run .session or -s without a session name.
2. Set save_session: true.

The autonamed session will be saved to <aichat-config-dir>/sessions/_/<datatime>-<autoname>.yaml

The autonamed session can be loaded with the name _/<datatime>-<autoname>

solve #947 #805, replace #811

[sudomain]

This sounds fantastic and something I would use by default when invoking aichat. Is it easy to resume the session using TAB/auto-completion? From the screenshot it seems that is YYYYMMDD... format. As the number of sessions in a given year increases, I imagine the autocomplete will quickly become unwieldy. Perhaps <autoname>-<datatime>.yaml would be better?

[sigoden]

The <datetime>-<autoname>.yaml makes more sense.Users typically only pay attention to sessions generated in the last few
days. Searching by date is more convenient than searching by name.