[codex] Add desktop reconnecting proxy troubleshooting

[toby-bridges]

Summary

• Add a user-facing Reconnecting troubleshooting section to the CodexGuide troubleshooting manual.
• Clarify the boundary between config.toml and ~/.codex/.env for Desktop/IDE environment variables.
• Link to OpenAI docs and the community fact-check guide for higher-fidelity follow-up.
• Add a short caution for macOS one-shot scripts: treat them as templates unless they back up .env, validate proxy protocol with curl -x, and avoid overwriting secrets.

Conflict resolution

• Rebases the PR branch onto the latest freestylefly/CodexGuide:main (9baae05).
• Keeps the Reconnecting troubleshooting section on the renamed upstream page docs/guide/19-troubleshooting.md.

Validation

• git diff --check origin/main...HEAD
• corepack pnpm build rendered 60 pages successfully
• gh pr checks 16 --repo freestylefly/CodexGuide shows only Vercel deployment authorization failure, not a build or merge conflict failure

[vercel]

@toby-bridges is attempting to deploy a commit to the canghe's projects Team on Vercel.

A member of the Team first needs to authorize it.