Make the /color session-name bar themeable (accept hex or follow the theme)

Problem

/color sets the color of the session-name bar (the colored separator
lines and the name box around the prompt input, shown when a session has
a name via --name or /rename). It offers only 8 fixed named colors
(red, blue, green, yellow, purple, orange, pink, cyan), plus default.

These named colors are independent of the active theme. A user with a
custom theme (~/.claude/themes/<name>.json) cannot make the session-name
bar match the theme's promptBorder accent: /color's blue/red are
fixed presets, not the theme's values, and there is no way to pass a hex
color.

An unnamed session's input border does use the theme promptBorder;
naming the session replaces that with the /color palette, so a named
session's prompt bar cannot be made to match a custom theme.

Requested feature

Any one of these would resolve it:

1. Let /color accept a hex value (e.g. /color #1e97fd).
2. Let /color accept a theme token name (e.g. /color promptBorder).
3. Have the session-name bar inherit the theme's promptBorder by

default, with /color as an override.

Environment

• Claude Code 2.1.145
• macOS