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.go

@@ -0,0 +1,43 @@
+package llm
+
+import "encoding/json"
+
+const tokenEstimateCharRatio = 4
+
+// EstimateTokens estimates the token cost of messages and tool definitions in a
+// chat completion request. It intentionally uses a simple JSON-size heuristic:
+// roughly four UTF-8 bytes per token plus a small per-message framing overhead.
+func EstimateTokens(messages []Message, tools []Tool) int {
+	if len(messages) == 0 && len(tools) == 0 {
+		return 0
+	}
+
+	payload := struct {
+		Messages []Message `json:"messages,omitempty"`
+		Tools    []Tool    `json:"tools,omitempty"`
+	}{
+		Messages: messages,
+		Tools:    tools,
+	}
+	data, err := json.Marshal(payload)
+	if err != nil {
+		return 0
+	}
+
+	return estimateBytesAsTokens(len(data)) + len(messages)*4
+}
+
+func estimateMessageTokens(msg Message) int {
+	data, err := json.Marshal(msg)
+	if err != nil {
+		return 0
+	}
+	return estimateBytesAsTokens(len(data)) + 4
+}
+
+func estimateBytesAsTokens(n int) int {
+	if n <= 0 {
+		return 0
+	}
+	return (n + tokenEstimateCharRatio - 1) / tokenEstimateCharRatio
+}