[codex] Enable remote plugins by default

[xl-openai]

Summary

• enable the remote plugin feature by default
• promote the remote plugin feature from under development to stable
• preserve the existing features.remote_plugin override for explicitly disabling it
• keep legacy disabled-path coverage explicit in TUI and app-server tests

Impact

Remote plugin functionality is enabled by default for configurations that do not set the feature flag. The existing Codex backend authentication gate still applies.

Validation

• just fmt
• just test -p codex-features
• just test -p codex-tui plugins_popup_remote_section_fallback_states_snapshot
• targeted codex-app-server plugin-list and skills-list tests
• git diff --check

The full TUI and app-server suites were also exercised locally. All remote-plugin-related coverage passed; unrelated local sandbox/test-binary failures remain outside this change.

[canvrno-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: 649eceeeaf

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