Feature proposal: configurable TUI prompt glyph and background

Summary

Add runtime configuration for the TUI prompt glyph and prompt background color.

Why

This makes user prompts:

• visually distinct from system and output text
• easier to identify at a glance
• easier to match with terminal tab/workspace colors in tools like Warp and similar terminals
• more usable across multiple concurrent sessions

This is especially helpful when several Codex sessions are open at once and the user wants prompt surfaces to visually align with terminal color coding.

Proposed Behavior

• tui.prompt_glyph controls the runtime prompt marker
• tui.prompt_background controls the prompt surface background
• defaults preserve current behavior when unset

Scope Implemented in My Fork

I already prototyped this in my fork and verified:

• config model and schema support
• runtime prompt glyph replacement in the TUI
• prompt background wired through shared styling
• matching behavior in mirrored tui_app_server paths
• tests passing after snapshot updates

Validation Performed

• cargo check -p codex-tui -p codex-tui-app-server -p codex-core
• cargo test -p codex-tui

Prototype Reference

• Branch: ggb667/codex:feat/tui-configurable-prompt
• Commit: a143db648a48441b38a77d531823d12c44fb219c

Request

If this aligns with the product direction, I would welcome feedback. If useful, I can also provide implementation notes, analysis, or a high-level patch outline from the working fork.

Example Use Case

codex-tui -c tui.prompt_glyph="🎈" -c tui.prompt_background="#FFEEEE"

🎈✶🍎🦋💎⚡🐲 - any unicode symbol or glyph.  Right next to data entry makes wrong panel very hard to do by mistake.

Proposed config

[tui]
prompt_glyph = "🎈"
prompt_background = "#FFEEEE"