Replaced `LatestConversationMessages` with `LoadConversationList`, which
utilizes `LastMessageAt` for much faster conversation loading in the
conversation listing TUI and `lmcli list` command.
This refactor splits out all conversation concerns into a new
`conversation` package. There is now a split between `conversation` and
`api`s representation of `Message`, the latter storing the minimum
information required for interaction with LLM providers. There is
necessary conversation between the two when making LLM calls.
Updated the behaviour of commands:
- `lmcli edit`
- by default create a new branch/message branch with the edited contents
- add --in-place to avoid creating a branch
- no longer delete messages after the edited message
- only do the edit, don't fetch a new response
- `lmcli retry`
- create a new branch rather than replacing old messages
- add --offset to change where to retry from
- Split pkg/cli/cmd.go into new pkg/cmd package
- Split pkg/cli/functions.go into pkg/lmcli/tools package
- Refactor pkg/cli/openai.go to pkg/lmcli/provider/openai
Other changes:
- Made models configurable
- Slight config reorganization
