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 ./...

README.md

@@ -20,13 +20,13 @@ model/provider switching, and local tools.
 mkdir -p ~/.agentu
 cp agentu.example.yaml ~/.agentu/config.yaml
 export AGENTU_API_KEY='your-api-key'
-go run ./cmd/agentu
+go run .
 ```
 
 Use `--yolo` only when you want agentu to write files or run shell commands:
 
 ```sh
-go run ./cmd/agentu --yolo
+go run . --yolo
 ```
 
 ## Config
@@ -82,7 +82,7 @@ top-level OpenAI-compatible request field named `thinking`.
 ## CLI Flags
 
 ```sh
-go run ./cmd/agentu [flags]
+go run . [flags]
 ```
 
 - `--config <path>` loads a YAML config file. Default: `~/.agentu/config.yaml`.
@@ -150,7 +150,7 @@ Run the verification suite:
 ```sh
 go test ./...
 go vet ./...
-go build ./cmd/agentu
+go build .
 ```
 
 Local config and build outputs are intentionally ignored by git.