Large refactor - it compiles!

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.

pkg/conversation/tools.go

@@ -0,0 +1,55 @@
+package conversation
+
+import (
+	"git.mlow.ca/mlow/lmcli/pkg/api"
+)
+
+// ApplySystemPrompt updates the contents of an existing system Message if it
+// exists, or returns a new slice with the system Message prepended.
+func ApplySystemPrompt(m []Message, system string, force bool) []Message {
+	if len(m) > 0 && m[0].Role == api.MessageRoleSystem {
+		if force {
+			m[0].Content = system
+		}
+		return m
+	} else {
+		return append([]Message{{
+			Role:    api.MessageRoleSystem,
+			Content: system,
+		}}, m...)
+	}
+}
+
+func MessageToAPI(m Message) api.Message {
+	return api.Message{
+		Role:        m.Role,
+		Content:     m.Content,
+		ToolCalls:   m.ToolCalls,
+		ToolResults: m.ToolResults,
+	}
+}
+
+func MessagesToAPI(messages []Message) []api.Message {
+	ret := make([]api.Message, 0, len(messages))
+	for _, m := range messages {
+		ret = append(ret, MessageToAPI(m))
+	}
+	return ret
+}
+
+func MessageFromAPI(m api.Message) Message {
+	return Message{
+		Role:        m.Role,
+		Content:     m.Content,
+		ToolCalls:   m.ToolCalls,
+		ToolResults: m.ToolResults,
+	}
+}
+
+func MessagesFromAPI(messages []api.Message) []Message {
+	ret := make([]Message, 0, len(messages))
+	for _, m := range messages {
+		ret = append(ret, MessageFromAPI(m))
+	}
+	return ret
+}