About shows the artwork in kitty and Ghostty, and Diagnostics says how it's drawn #537

Context

About (tui) draws its artwork as a sixel, so kitty and Ghostty, which don't speak sixel, get the ASCII art. Terminal.Gui 2.5's ImageView draws through kitty graphics as well as sixel. This task moves About onto it, keeping our own probe for what TG's detection gets wrong, and keeping the ASCII art where neither protocol is available. Pitch: #164.

Acceptance criteria

• In kitty, tui shows the artwork, not the ASCII art.
• In Ghostty, tui shows the artwork, not the ASCII art.
• In WezTerm and iTerm2, About looks as it does today: the image is the right size on a Retina display, with no dark band under it in iTerm2.
• Inside tmux 3.6 (in a sixel terminal), About still shows the artwork.
• In Alacritty or Terminal.app, About shows the ASCII art, unchanged.
• Opening About a second time shows the image straight away, and closing it leaves nothing behind on screen, in each terminal above.
• F12 Diagnostics has an Image protocol line reading kitty graphics, sixel or none (ASCII art), matching what About drew.

Tests

• Choosing the protocol: kitty graphics when TG detects kitty/Ghostty (outside tmux), sixel when our probe finds it, otherwise ASCII art; inside tmux, never kitty graphics.
• Our probe's sixel result and cell size are what the driver is given, so TG's own detection doesn't override them.
• The Diagnostics line for each of the three outcomes.
• AGENTS.md's About dialog section updated to describe the new path; the sixel drawing code ImageView replaces is removed.

Out of scope

• ImageView's coloured-cell fallback: terminals with no image protocol keep the ASCII art.
• Kitty graphics inside tmux (passthrough).
• Images anywhere else, such as previewing image files.
• ImageView's pan and zoom keys.
• A probe of our own for kitty graphics over SSH.