Saga skill: what it does and how to install it

What it does. Turns a feature request into a spec tree under ~/.sagas/, covering milestones, task contracts, dependencies and validation criteria. It delegates implementation in parallel worktrees, tracks progress, integrates milestones, runs final checks and asks for manual acceptance.

When to use it. For medium-to-large features that need autonomous, end-to-end implementation with limited human intervention, or to resume an existing saga from its progress directory. Use it when requirements can be made explicit enough for workers to validate their own changes.