Refresh README documentation

[danielitoCode]

Motivation

• Modernize and professionalize the README to reflect the current Core V4 scope and capabilities.
• Improve clarity around the deterministic generation model, the 5-stage pipeline, and dry-run/preview behavior.
• Provide a concise product overview and a clearer section structure for contributors and users.

Description

• Replaced the existing README with a streamlined, reorganized document that groups content into Highlights, Core Philosophy, Current State, What Core V4 Includes, Generation Pipeline, Preview & Dry-Run, Internal Architecture, Testing & Quality, Out of Scope, Roadmap (Core V5), and Running instructions.
• Clarified the generation pipeline as five explicit stages and described dry-run behavior and UI previews.
• Simplified wording, removed redundant text, and improved headings and formatting for readability.
• Kept the existing run command example (./gradlew :composeApp:run) and preserved scope boundaries in the documentation.

Testing

• No automated tests were run because this is a documentation-only change.
• The change is expected to be CI-neutral and should not affect builds or runtime behavior.

---

Codex Task