docs(sixel): hard-state img2sixel as the recommended encoder + warn loudly when none is on PATH

Users hit a silent-failure mode where `kitty-gfx--sixel-detect' returned
true (terminal supports Sixel) but no encoder was on `PATH', so every
`sixel-place' logged `no encoder found' and produced no image.  The
echo-area `message' from `sixel-encode' was easy to miss inside tmux,
which made the issue look like a refresh / tmux passthrough bug.

- README.org: Requirements section now explicitly lists a Sixel encoder
  as required and recommends `img2sixel' (libsixel) over ImageMagick,
  with the concrete numbers from PR #10's bench (~2.2x smaller payload,
  ~10x faster) and a note that this matters most inside tmux because
  every refresh re-emits the full DCS payload.  Known-Limitations
  reworded: missing encoder = silent failure surfaced via the debug log.
- AGENTS.md: Dependencies block reorganised the same way; adds a
  pointer to `just sixel-encoder' as the first step when diagnosing
  Sixel rendering bugs.
- kitty-graphics.el: when the active backend is Sixel and
  `kitty-gfx--sixel-resolve-encoder' returns nil at mode-enable time,
  fire a `display-warning' (`*Warnings*' buffer + visible message)
  pointing at img2sixel/ImageMagick, so the failure is no longer
  buried in /tmp/kitty-gfx.log.

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