feat(sixel): configurable encoder + per-invocation timeout

Sixel encoding was hard-coded to ImageMagick (`magick'/`convert') with no
timeout, so a malformed image or hung encoder could freeze Emacs and
users could not switch to libsixel's faster `img2sixel'.

- New customs: `kitty-gfx-sixel-encoder-program' (nil = auto-detect:
  prefer img2sixel, then magick, then convert), `-encoder-args' (extra
  args list), `-encoder-timeout' (float seconds, default 5.0, nil =
  no timeout).
- New helper `kitty-gfx--sixel-resolve-encoder' returns
  (KIND . ABS-PATH) where KIND is `img2sixel' or `imagemagick' so the
  encode pipeline can build the right argv (`-w/-h <px>' vs
  `-geometry WxH sixel:-').
- New helper `kitty-gfx--sixel-run-encoder' runs the encoder via
  `make-process' with a separate stderr buffer and a `run-at-time'
  watchdog that kills the process on timeout; failures and timeouts
  are logged and surface as nil from `kitty-gfx--sixel-encode'
  rather than freezing Emacs.
- Existing magick/convert path preserved and still chosen automatically
  when img2sixel is absent.
- AGENTS.md: dependency note updated to mention img2sixel and the new
  customs.

Inspired by and co-authored with @timfel's sixel-graphics.el fork
(https://github.com/timfel/sixel-graphics.el), which introduced the
configurable encoder + timeout pattern.

Refs #4.

Co-authored-by: Tim Felgentreff <timfel@users.noreply.github.com>

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