[codex] Point codex cron sidecar at Obsidian root

[boxp]

Summary

• Change the Codex workspace scheduler sidecar CODEX_CRON_ROOT from ~/.codex-cron to /home/boxp/Documents/obsidian-headless/BOXP/Infrastructure/Codex Cron.
• Update argoproj/codex-workspace/cron.md so the documented live registry, prompts, and runs paths match the new direct Obsidian root.
• Add the project plan under docs/project_docs/codex-cron-obsidian-root/plan.md.

Why

The scheduler currently receives an explicit CODEX_CRON_ROOT=/home/boxp/.codex-cron, so changing only the image default would leave the live sidecar reading the symlink. This PR makes the deployed scheduler point at the Obsidian vault directory directly.

Validation

• Path references checked with rg.
• kubectl and kustomize are not installed in the current workspace, so manifest build was not run here.

[github-actions]

ArgoCD Diff Result

Auth path: tailscale

アプリケーション: codex-workspace の差分

パス: argoproj/codex-workspace

===== apps/Deployment codex-workspace/codex-workspace ======
681c681,682
<           value: /home/boxp/.codex-cron
---
>           value: /home/boxp/Documents/obsidian-headless/BOXP/Infrastructure/Codex
>             Cron
ℹ️ 上記の差分が見つかりました