fix: make CODEX_HOME resolution consistent

[hosmelq]

Summary

• Centralize Codex home resolution so config, prompts, and session data all use the same CODEX_HOME/default path.
• Ensure workspace codex home falls back to the global default when no .codexmonitor override exists.
• Remove divergent CODEX_HOME resolvers that caused the UI and app-server to read from different locations.

Why

Locally I set CODEX_HOME to ~/.config/codex, but the app wasn’t loading it because global paths (prompts/config/session data) and workspace spawns didn’t share the same Codex home. That left the app-server reading a different location than the UI, resulting in empty project sessions. This change makes CODEX_HOME authoritative everywhere and applies it consistently so sessions and config load as expected.

Testing

• npm run test
• npm run lint
• npm run typecheck

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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