refactor: simplify to thin picker architecture

[alextnetto]

Summary

Complete rewrite from persistent TUI to thin picker.

Before

TUI stays open → runs async tmux commands → refreshes → handles errors inline
→ Complex state machine, mock objects, input bugs in tmux popup

After

Fetch sessions → show picker → user picks action → TUI exits → run tmux command
→ ~300 lines total, zero input handling bugs

Removed (-1,983 lines)

• internal/setup/ — auto-setup prompt, --setup flag
• tmux.Client interface, MockClient, integration tests
• Async command pattern (sessionsMsg, operationMsg)
• SetupFunc injection

Added (+388 lines)

• tui.Action struct — TUI returns intent, caller executes
• Package-level tmux.* functions — no interface indirection
• 21 focused tests (was 60+, now testing only what matters)

Architecture

main.go:
  sessions := tmux.ListSessions()    // 1. fetch
  model := tui.New(sessions)          // 2. show picker
  action := model.Action()            // 3. get user intent
  execute(action)                     // 4. run tmux command