Clarify AI-guided migration identity

[leesunghyun]

Summary

This PR updates repository identity documentation after the repo rename and workflow expansion:

• Clarify README identity as AI-guided migration lab
• Add Motivation, What this project is / is not, Core scenario, AI-guided workflow
• Update Quick Start URLs and OSS release links to new repo slug
• Split archive docs from migration docs
• Update docs/codex-for-oss.md with explicit AI+human PR-style safeguards
• Update task-plan clone command to the new repository URL
• Add v0.2.0 roadmap and release note placeholders
• Polish MainActivity preview examples to use real sample screens

Verification

• Branch changes committed and pushed
• No code behavior was introduced/removed in core runtime paths; this PR is documentation-focused with preview cleanup only.