fix: address Plan 2 final-review findings

- tui/doc.go: remove teatest blank import (test harness in prod binary)
- cmd: honor wizard HaltOnError toggle in addition to --halt-on-error flag
- cmd: wire SIGINT/SIGTERM to root context via signal.NotifyContext
- dashboard/run: derive engine context and cancel on tea.Program exit
  so the engine goroutine does not outlive Run

internal/tui/doc.go

@@ -2,10 +2,3 @@
 // that collects user choices and the Bubble Tea dashboard that renders
 // live module status during engine.Run.
 package tui
-
-import (
-	_ "github.com/charmbracelet/bubbletea"
-	_ "github.com/charmbracelet/huh"
-	_ "github.com/charmbracelet/lipgloss"
-	_ "github.com/charmbracelet/x/exp/teatest"
-)