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 ./...

internal/session/manager.go

@@ -8,8 +8,8 @@ import (
 	"time"
 
 	"agentu/internal/agent"
-	"agentu/internal/config"
-	"agentu/internal/llm"
+	"agentu/pkg/config"
+	"agentu/pkg/llm"
 )
 
 type Manager struct {

internal/session/manager_test.go

@@ -7,8 +7,8 @@ import (
 	"testing"
 
 	"agentu/internal/agent"
-	"agentu/internal/config"
-	"agentu/internal/llm"
+	"agentu/pkg/config"
+	"agentu/pkg/llm"
 )
 
 type recordingProvider struct {

internal/session/store.go

@@ -11,7 +11,7 @@ import (
 	"strings"
 	"time"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 )
 
 // Session represents a persisted conversation.

internal/session/store_test.go

@@ -5,7 +5,7 @@ import (
 	"path/filepath"
 	"testing"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 )
 
 func tempStore(t *testing.T) *Store {