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/conversations/conversations.go

@@ -5,7 +5,7 @@ import (
 	"strings"
 	"time"
 
-	models "git.mlow.ca/mlow/lmcli/pkg/lmcli/model"
+	"git.mlow.ca/mlow/lmcli/pkg/api"
 	"git.mlow.ca/mlow/lmcli/pkg/tui/shared"
 	"git.mlow.ca/mlow/lmcli/pkg/tui/styles"
 	tuiutil "git.mlow.ca/mlow/lmcli/pkg/tui/util"
@@ -16,15 +16,15 @@ import (
 )
 
 type loadedConversation struct {
-	conv      models.Conversation
-	lastReply models.Message
+	conv      api.Conversation
+	lastReply api.Message
 }
 
 type (
 	// sent when conversation list is loaded
 	msgConversationsLoaded ([]loadedConversation)
 	// sent when a conversation is selected
-	msgConversationSelected models.Conversation
+	msgConversationSelected api.Conversation
 )
 
 type Model struct {