Explain Codex task provider persistence

[swaminator]

Summary

• explain that Codex persists the model provider when each task is created
• require a new task after installing or reconnecting before selecting a Webster model
• print the guidance for managed, unmanaged, already-restarted, and stopped app-server states
• document the ChatGPT-account error produced when an older OpenAI task selects a Webster model

Validation

• bash -n install.sh codex-setup.sh test.sh

• ./test.sh

• npm test in codex-model-proxy

• shellcheck codex-setup.sh

• git diff --check

• actionlint .github/workflows/bootstrap.yml

• This change includes validation that would fail if the new behavior broke.

• Relevant success and failure paths are tested.

• Safe re-run/idempotency behavior is preserved.

• The supported GitHub Actions matrix passes.

Documentation

• User-facing documentation was updated, or no documentation change is needed.