Phase 11 Devlog: README

Implemented

Rewrote README.md around the implemented MVP rather than the earlier planned MVP.
Explained what NightShift is and what it is not.
Added development install and direct module usage.
Added quickstart commands for init, validate, run, and run --task.
Added task file and config examples that match the current command-backed MVP.
Documented command-backed agent behavior and review output contracts.
Documented the current safety model.
Documented the artifact layout created by the runner.
Added testing commands and a concise roadmap.

Kept README focused on user-facing operation and reviewability instead of implementation internals.
Described PyYAML as optional because the MVP has a small standard-library fallback parser for starter configs.
Left future backend details in the roadmap rather than implying they already exist.