feat: add Atlas Cloud upscale provider

[binyangzhu000-sudo]

What

• add Atlas Cloud as an optional provider to the existing fal-upscale skill
• submit Atlas generation once, then poll the prediction with bounded backoff
• document provider setup and cover Atlas, failed-submit, and legacy fal paths

Why

This gives image-upscaling users a BYOK provider choice while keeping fal.ai as
the default and leaving the existing video models on fal.ai.

How to test

skills/fal-upscale/tests/test_upscale.sh
uvx ruff format --check .
uvx ruff check .
python3.11 -m compileall -q app agent_core agents decorators skills
uvx --from shellcheck-py shellcheck -e SC1091 \
  skills/fal-upscale/scripts/upscale.sh \
  skills/fal-upscale/tests/test_upscale.sh

The live Atlas model catalog and schema were checked for
atlascloud/image-upscaler. A single live submit was attempted without retry;
it returned HTTP 402 (insufficient balance), so no prediction was created.

Screenshots / Logs

Not applicable; this is a shell skill integration.

[zfoong]

@binyangzhu000-sudo is this PR ready to be reviewed?