fix(sixel): accept tmux/screen TERM when tmux forwards Sixel (#14)

`kitty-gfx--sixel-detect' currently gates on the outer TERM matching
`xterm|vt[0-9]|foot|contour'.  Once `tmux-ok' has confirmed tmux >=
3.4, tmux itself owns the Sixel rendering -- the DCS payload never
reaches the outer terminal.  At that point the outer TERM regex is
irrelevant, but the canonical tmux assignment `tmux-256color' (and
the legacy `screen-256color') still trip the check, and detection
returns nil even when Sixel works fine.

Accept `tmux*' and `screen*' TERMs when `tmux-ok' is true.  Outside
tmux the existing regex still rules, so no behavioural change for
non-tmux setups.

Repro before this patch:

    TERM=tmux-256color emacs -nw -Q -l kitty-graphics.el \
        --eval '(setq kitty-gfx-debug t)' \
        --eval '(kitty-graphics-mode 1)'
    => "Kitty graphics: terminal not supported"
    *kitty-gfx-debug*: sixel-detect: nil (TERM=tmux-256color ... tmux-ok=yes)

1 files changed, 6 insertions, 0 deletions

diff --git a/kitty-graphics.el b/kitty-graphics.el
index d1514df..4879de6 100644
--- a/kitty-graphics.el
+++ b/kitty-graphics.el
@@ -650,6 +650,12 @@ Sixel because they drop the DCS payload."
          (supported (and term
                          (or (not in-tmux) tmux-ok)
                          (or (string-match-p "xterm\\|vt[0-9]\\|foot\\|contour" term)
+                             ;; Once `tmux-ok' is true, tmux >= 3.4 itself
+                             ;; renders Sixel, so the outer TERM regex is
+                             ;; irrelevant -- accept the canonical
+                             ;; tmux-* / screen-* TERMs that tmux assigns.
+                             (and tmux-ok
+                                  (string-match-p "\\`\\(tmux\\|screen\\)\\b" term))
                              (member term-prog '("foot" "Konsole" "mintty" "mlterm"
                                                  "contour" "WezTerm"))
                              windows-terminal)