refactor: restructure project layout — main.go to root, llm+config to pkg/

- Move cmd/agentu/main.go → main.go (project root)
- Move internal/llm/ → pkg/llm/ (leaf package, no internal deps)
- Move internal/config/ → pkg/config/ (leaf package, no internal deps)
- Update all import paths: agentu/internal/llm → agentu/pkg/llm (14 files)
- Update all import paths: agentu/internal/config → agentu/pkg/config (3 files)
- Update README: build/run commands, CLI flags section
- agent/session/tools/tui stay in internal/ (app-specific business logic)

Build: go build . | Test: go test ./... | Vet: go vet ./...

pkg/llm/types.go

@@ -0,0 +1,97 @@
+package llm
+
+import (
+	"context"
+	"encoding/json"
+)
+
+const (
+	RoleSystem    = "system"
+	RoleUser      = "user"
+	RoleAssistant = "assistant"
+	RoleTool      = "tool"
+)
+
+type Provider interface {
+	ChatStream(ctx context.Context, req ChatRequest, emit func(StreamEvent) error) error
+}
+
+type ChatRequest struct {
+	Model      string         `json:"model"`
+	Messages   []Message      `json:"messages"`
+	Tools      []Tool         `json:"tools,omitempty"`
+	ToolChoice string         `json:"tool_choice,omitempty"`
+	Stream     bool           `json:"stream"`
+	Extra      map[string]any `json:"-"`
+}
+
+func (r ChatRequest) MarshalJSON() ([]byte, error) {
+	payload := map[string]any{
+		"model":    r.Model,
+		"messages": r.Messages,
+		"stream":   r.Stream,
+	}
+	if len(r.Tools) > 0 {
+		payload["tools"] = r.Tools
+	}
+	if r.ToolChoice != "" {
+		payload["tool_choice"] = r.ToolChoice
+	}
+	for key, value := range r.Extra {
+		if _, exists := payload[key]; exists {
+			continue
+		}
+		payload[key] = value
+	}
+	return json.Marshal(payload)
+}
+
+type Message struct {
+	Role       string     `json:"role"`
+	Content    string     `json:"content,omitempty"`
+	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
+	ToolCallID string     `json:"tool_call_id,omitempty"`
+}
+
+type ToolCall struct {
+	ID       string       `json:"id,omitempty"`
+	Type     string       `json:"type,omitempty"`
+	Function FunctionCall `json:"function,omitempty"`
+}
+
+type FunctionCall struct {
+	Name      string `json:"name,omitempty"`
+	Arguments string `json:"arguments,omitempty"`
+}
+
+type Tool struct {
+	Type     string       `json:"type"`
+	Function ToolFunction `json:"function"`
+}
+
+type ToolFunction struct {
+	Name        string          `json:"name"`
+	Description string          `json:"description"`
+	Parameters  json.RawMessage `json:"parameters"`
+}
+
+type StreamEvent struct {
+	Content      string
+	ToolCalls    []ToolCallDelta
+	FinishReason string
+	Usage        *Usage
+}
+
+type Usage struct {
+	PromptTokens     int `json:"prompt_tokens"`
+	CompletionTokens int `json:"completion_tokens"`
+	TotalTokens      int `json:"total_tokens"`
+}
+
+type ToolCallDelta struct {
+	Index     int
+	ID        string
+	Type      string
+	Name      string
+	Arguments string
+}