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/tools/code.go

@@ -16,7 +16,7 @@ import (
 	"sort"
 	"strings"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 )
 
 const defaultCodeSymbolFileLimit = 50

internal/tools/file.go

@@ -13,7 +13,7 @@ import (
 	"sort"
 	"strings"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 )
 
 type FileReadTool struct {

internal/tools/shell.go

@@ -9,7 +9,7 @@ import (
 	"os/exec"
 	"strings"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 )
 
 type ShellRunTool struct {

internal/tools/tools.go

@@ -7,7 +7,7 @@ import (
 	"sort"
 	"strings"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 )
 
 const MaxToolOutputBytes = 64 * 1024

internal/tools/web_builtin.go

@@ -10,7 +10,7 @@ import (
 	"net/url"
 	"strings"
 
-	"agentu/internal/llm"
+	"agentu/pkg/llm"
 
 	"golang.org/x/net/html"
 )