Package restructure and API changes, several fixes

- More emphasis on `api` package. It now holds database model structs
  from `lmcli/models` (which is now gone) as well as the tool spec,
  call, and result types. `tools.Tool` is now `api.ToolSpec`.
  `api.ChatCompletionClient` was renamed to
  `api.ChatCompletionProvider`.

- Change ChatCompletion interface and implementations to no longer do
  automatic tool call recursion - they simply return a ToolCall message
  which the caller can decide what to do with (e.g. prompt for user
  confirmation before executing)

- `api.ChatCompletionProvider` functions have had their ReplyCallback
  parameter removed, as now they only return a single reply.

- Added a top-level `agent` package, moved the current built-in tools
  implementations under `agent/toolbox`. `tools.ExecuteToolCalls` is now
  `agent.ExecuteToolCalls`.

- Fixed request context handling in openai, google, ollama (use
  `NewRequestWithContext`), cleaned up request cancellation in TUI

- Fix tool call tui persistence bug (we were skipping message with empty
  content)

- Now handle tool calling from TUI layer

TODO:
- Prompt users before executing tool calls
- Automatically send tool results to the model (or make this toggleable)

pkg/tui/views/chat/input.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"strings"
 
-	models "git.mlow.ca/mlow/lmcli/pkg/lmcli/model"
+	"git.mlow.ca/mlow/lmcli/pkg/api"
 	"git.mlow.ca/mlow/lmcli/pkg/tui/shared"
 	tuiutil "git.mlow.ca/mlow/lmcli/pkg/tui/util"
 	tea "github.com/charmbracelet/bubbletea"
@@ -150,12 +150,12 @@ func (m *Model) handleInputKey(msg tea.KeyMsg) (bool, tea.Cmd) {
 			return true, nil
 		}
 
-		if len(m.messages) > 0 && m.messages[len(m.messages)-1].Role == models.MessageRoleUser {
+		if len(m.messages) > 0 && m.messages[len(m.messages)-1].Role == api.MessageRoleUser {
 			return true, shared.WrapError(fmt.Errorf("Can't reply to a user message"))
 		}
 
-		m.addMessage(models.Message{
-			Role:    models.MessageRoleUser,
+		m.addMessage(api.Message{
+			Role:    api.MessageRoleUser,
 			Content: input,
 		})