docs: embedded-first getting started — same business logic everywhere (#158)

[enricopiovesan]

Summary

Add docs/getting-started-embedded.md — a guided embedded-first path that teaches the WASM-once / UI-shell model on platforms that already embed (Web React, Linux GTK, Rust CLI). Root README.md Getting Started now leads with that path and demotes traverse-cli serve to a clearly labeled Dev sidecar (Phase 1/2 interim) subsection for Apple / Android / Windows.

Cross-links from docs/embedded-runtime-plan.md, docs/traverse-starter-plan.md, docs/youaskm3-starter-kit.md, and apps/youaskm3-starter-kit/README.md.

Governing Spec

• UI-only constitution: docs do not add business-field computation or private Traverse imports
• Phase 3 architecture: docs/embedded-runtime-plan.md
• Public embedder surfaces only (traverse-embedder-web / traverse-embedder)

Project Item

• Issue: Docs: embedded-first getting started — build an app with the same business logic everywhere #158
• Project 2: Status In Progress; Agent Cursor

Definition of Done

• docs/getting-started-embedded.md covers mental model, clone/install, embedded Web, Linux/CLI, business-vs-UI edit guide, add-platform checklist, and platform matrix with Embed WASM runtime: Swift clients (iOS + macOS, traverse-starter + doc-approval) #114–Embed WASM runtime: Windows WinUI clients (traverse-starter + doc-approval) #116 links
• Root README Getting Started leads with embedded; sidecar is interim subsection
• Plan docs + youaskm3 starter kit link the new guide
• Doc-only; no UI business logic or runtime fakes

Validation

test -f docs/getting-started-embedded.md
bash scripts/ci/repository_checks.sh   # PASS

Checklist

• Issue linked and Project 2 item updated
• Updated AGENTS.md platform table if this changes platform status (N/A — onboarding docs only)
• No business logic added to UI layer
• CI gates pass locally where applicable