Fix TUI channel editor save UX

[edwin-zvs]

Summary

• make C-s the only save key in the TUI channel editor; Enter no longer saves
• track daemon-confirmed channel state and render a dirty * until save succeeds
• show an explicit C-s save hint and cover dirty, rendering, save, clean-state, and Enter behavior
• record the channel save convention in the existing operator-view UX spec

Tests

• cargo test -p construct-cli channel_editor_
• cargo test -p construct-cli slack_personal_tui_editor_sends_response_mode_overrides_on_save
• cargo test -p construct-cli a_channel_saved_without_typing_adopts_its_id_placeholder
• cargo build --workspace --bins