tmux: hardcoded chalk level downgrade ignores truecolor-capable terminals

Bug Description

Claude Code unconditionally downgrades chalk from level 3 (truecolor/16M) to level 2 (256-color) when running inside tmux, regardless of whether the tmux session actually supports truecolor. This causes all UI colors to appear muted/toned-down compared to running in a raw terminal.

Root Cause

The ink2 renderer contains a hardcoded function (introduced in 2.1.77 as part of "Fixed background colors rendering as terminal-default inside tmux"):

function HYK() {
  if (process.env.TMUX && KT.level > 2)
    return KT.level = 2, true;  // unconditionally caps to 256-color
  return false;
}

This runs after FORCE_COLOR detection, overriding it. Setting FORCE_COLOR=3 in settings.json env block has no effect because the tmux downgrade fires later.

Impact

• All hex colors (e.g., Claude orange #FF6600) get approximated to the nearest 256-color palette match, resulting in visibly duller/muted colors
• The FORCE_COLOR=3 env var workaround does not work because the downgrade runs after initial chalk level detection
• Affects all tmux users, including those with terminals that fully support truecolor through tmux (Ghostty, iTerm2, Kitty, WezTerm with Tc terminal-override)

Environment

• Terminal: Ghostty (supports truecolor natively)
• tmux: 3.6a with set -ga terminal-overrides ",xterm-ghostty:Tc,tmux-256color:Tc"
• COLORTERM: truecolor (set correctly inside tmux)
• tmux client_termfeatures: bpaste,ccolour,clipboard,cstyle,focus,RGB,title (RGB confirmed)
• Truecolor passthrough: Verified working — smooth 24-bit gradient renders correctly in tmux
• Claude Code versions affected: 2.1.77 through 2.1.80 (current)
• Last working version: 2.1.76

Steps to Reproduce

1. Open Ghostty (or any truecolor terminal)
2. Start tmux with truecolor enabled (Tc flag in terminal-overrides)
3. Launch Claude Code inside tmux — colors appear muted
4. Launch Claude Code outside tmux (raw Ghostty) — colors are vibrant and correct
5. Compare side by side — tmux version is noticeably toned down

Expected Behavior

Claude Code should render full truecolor (chalk level 3) inside tmux when the terminal supports it. The function should check COLORTERM=truecolor or tmux's Tc capability before downgrading, rather than blanket-capping all tmux sessions to 256-color.

Suggested Fix

Replace the unconditional tmux downgrade:

// Current (broken for truecolor-capable terminals)
if (process.env.TMUX && KT.level > 2) return KT.level = 2, true;

// Suggested (respect COLORTERM and FORCE_COLOR)
if (process.env.TMUX && KT.level > 2 
    && process.env.COLORTERM !== 'truecolor' 
    && process.env.COLORTERM !== '24bit'
    && process.env.FORCE_COLOR !== '3') 
  return KT.level = 2, true;

Or better: check tmux's terminal features for RGB/Tc support before downgrading.