CLI: add /color for per-session input-bar colors

What variant of Codex are you using?

Codex CLI (codex-cli 0.144.1) on macOS.

What feature would you like to see?

Please add a /color [color|default] command that changes the input/composer bar color for the current Codex session, similar to Claude Code.

Suggested behavior:

• /color red|blue|green|yellow|purple|orange|pink|cyan
• /color default resets the color
• /color without an argument selects a random color
• The selected color is stored per thread and restored when the session is resumed
• The color changes the composer border/accent, not the global syntax-highlighting theme

I often run multiple Codex CLI sessions in different terminal tabs or panes. Giving each session a different input-bar color makes it immediately clear which task I am typing into and helps prevent prompts or commands from being sent to the wrong session.

Additional information

Claude Code reference behavior: https://code.claude.com/docs/en/commands

Related but distinct requests:

• #21130 requests broad semantic TUI color configuration.
• #15278 requested configurable prompt glyph/background settings.

This request is narrower: it adds a quick, per-session visual identity command rather than changing the global theme. The implementation should preserve readable contrast and respect terminal color capabilities and NO_COLOR.