[codex] Add Codex environment setup

[hopeatina]

Summary

• Add checked-in Codex cloud setup and maintenance scripts under .codex/.
• Add Codex Desktop local worktree setup and cleanup wrappers using CODEX_WORKTREE_PATH.
• Add repo-local agent guidance for setup and verification commands.
• Keep setup secrets out of source and document where live credentials are required.

Verification

• bash -n .codex/*.sh
• git diff --check
• bash .codex/setup-cloud.sh from a clean worktree for this repo

Rollout notes

No runtime migrations or production environment changes. These scripts affect Codex cloud/local worktree bootstrapping only.