Add selection quote replies experimental feature

[harish-oai]

Why/Context/Summary

Changes:

• add selection_quote_replies to the shared experimental feature registry with beta-stage metadata
• allow app-server experimental feature enablement writes for selection_quote_replies
• update app-server tests and regenerate the config schema fixture so the new feature key is represented end to end

Test plan

• cargo test -p codex-features
• cargo test -p codex-app-server --test all experimental_feature_list -- --nocapture --test-threads=1
• cargo test -p codex-core config_schema_matches_fixture

[github-actions]

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