Refactor platform control boundaries

[Che-Zhu]

Summary

• split project creation responsibilities across platform control, persistence, and integration boundaries
• add focused platform helper modules for GitHub installation lookup, repo access checks, namespace resolution, and clone task creation
• update platform architecture docs, PRD notes, and repository guidance for the new file and directory conventions

Validation

• pnpm lint
• pnpm exec tsc --noEmit

[github-actions]

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  Commit: e00768d46cf07db3a2f1180152828458853079a5
  Branch: codex/import-command-boundary

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