Add task-list replacement workflow

[kcosr]

Summary

• add prepare-only setTasks replacement support and the built-in task-list prepare hook
• load repo-local .task-runner/tasks.yml files into frozen run tasks, rebuilt task-local hooks, manifests, reset seeds, briefs, and assignment seeds
• document the container task-list workflow with host/container cwd split, command-based clone/update, aw-tr-launch usage, and repo-local hook trust notes

Validation

• npm run check: exit 0
• internal review run e5m92p: APPROVED for ship

Planning

• planning run: mdyr1n
• implementation run: 7st7t3

Accepted residuals

• LOW: assignment-seed snapshots for replaced tasks omit task-local hooks while manifest.resolvedHooks remains source of truth
• LOW: task-list missing-file detection checks the immediate cause only