[codex] Remove workspace owner usage nudge gate

[richardopenai]

Summary

• make workspace owner nudge handling unconditional in the TUI now that it is fully rolled out
• keep workspace_owner_usage_nudge as a removed no-op compatibility flag so old configs/app overrides remain accepted during rollout
• remove flag-disabled test setup

Companion PR

• https://github.com/openai/openai/pull/876351 removes the Codex Apps Statsig rollout gate override after this change is available to the app/runtime path

Validation

• just write-config-schema
• just fmt
• cargo test -p codex-features
• cargo test -p codex-tui status_and_layout

[chatgpt-codex-connector]

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