Why

! shell commands are currently surfaced as "Bash mode", which is misleading for users running shells such as PowerShell or zsh. Those commands also bypass the persistent prompt history path, so they cannot be recalled after starting a new session.

Fixes #19613.

What changed

• Rename the TUI footer label and related test wording from "Bash mode" to "Shell mode".
• Persist accepted ! shell commands to prompt history with the leading !, so recall restores the composer into shell mode across sessions.
• Add coverage for immediate and queued shell-command submissions emitting the prompt-history update.

Verification

• cargo test -p codex-tui bang_shell
• cargo test -p codex-tui shell_command_uses_shell_accent_style
• cargo test -p codex-tui footer_mode_snapshots
• cargo insta pending-snapshots --manifest-path tui/Cargo.toml

Manually verified fix after confirming presence of bug prior to fix.