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/token_estimate_test.go

@@ -0,0 +1,45 @@
+package llm
+
+import "testing"
+
+func TestEstimateTokensEmpty(t *testing.T) {
+	if got := EstimateTokens(nil, nil); got != 0 {
+		t.Fatalf("EstimateTokens(nil, nil) = %d, want 0", got)
+	}
+}
+
+func TestEstimateTokens(t *testing.T) {
+	shortMessages := []Message{{Role: RoleUser, Content: "hello"}}
+	short := EstimateTokens(shortMessages, nil)
+	if short <= 4 {
+		t.Fatalf("short estimate = %d, want message framing plus content", short)
+	}
+	if short > 50 {
+		t.Fatalf("short estimate = %d, looks too high for one short message", short)
+	}
+
+	longMessages := []Message{
+		{Role: RoleUser, Content: "hello"},
+		{Role: RoleAssistant, Content: "This is a longer response with enough content to exceed the short estimate."},
+	}
+	long := EstimateTokens(longMessages, nil)
+	if long <= short {
+		t.Fatalf("long estimate = %d, want > short estimate %d", long, short)
+	}
+
+	withTools := EstimateTokens(shortMessages, []Tool{{
+		Type: "function",
+		Function: ToolFunction{
+			Name:        "file_read",
+			Description: "Read a file",
+			Parameters:  []byte(`{"type":"object"}`),
+		},
+	}})
+	if withTools <= short {
+		t.Fatalf("withTools estimate = %d, want > short estimate %d", withTools, short)
+	}
+
+	if msgTokens := estimateMessageTokens(shortMessages[0]); msgTokens <= 4 {
+		t.Fatalf("estimateMessageTokens = %d, want message framing plus content", msgTokens)
+	}
+}