fix(sixel): detect Windows Terminal and route GIFs through image pipeline

Windows Terminal does not expose a stable `TERM' value, so the previous
sixel-detect check missed it.  Accept the `WT_SESSION', `WT_PROFILE_ID'
and `WT_WINDOWID' environment markers it injects into every session.

Also add `gif' to `kitty-gfx--image-file-p' so GIF files are routed
through the image pipeline.  Animation is still not supported -- only
the first frame is rendered -- but that is better than dropping them
on the floor.

AGENTS.md: list Windows Terminal under supported sixel terminals and
soften the GIF wording from "not supported" to "first frame only".

Closes #6.

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

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