feat(ui): add /prompt-color command for custom user input echo color - #2110

Motivation

In the current terminal theme, user input and model replies share nearly identical foreground colors with no visual delimiter between turns. During long sessions this makes it hard to quickly scan for human prompts. Allowing users to style their own echo(color&bold) creates a clear visual boundary and improves readability.

The implementation reuses Rich Style, so passing bold green achieves both boldface and color tint in one setting without extra fields.

Changes

• config.py — add prompt_text_color field
• theme.py — add runtime state + getter/setter for user echo color
• echo.py — apply Rich style to user text in render_user_echo* helpers
• ui/shell/__init__.py — sync color from runtime config on shell startup
• slash.py — introduce /prompt-color with Rich Style validation, config persistence, and hot-reload

Design Notes

• Accepts any valid Rich Style string (e.g. bold cyan, #ff6600) — no custom parser needed
• Hot-reload: changes take effect immediately without restarting the session (unlike /theme, which triggers a Reload)
• Empty default preserves backward compatibility

Usage

/prompt-color bold cyan
/prompt-color #ff6600
/prompt-color reset
Verification
• ruff check / ruff format ✅
• pyright ✅
• pytest (config + theme + echo) 56 passed ✅