v1.0.0-beta.1: docs: reorganize documentation structure (#21)

* feat: add landing page for beta onboarding

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: update GitHub links to genni613/CodePlanGUI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: reorganize docs into specs/design/planning folders, add roadmap and landing page updates

* fix: pr-agent.yml

---------

Co-authored-by: yuang.peng <yuang.peng@yaduo.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>