* docs: have agents put screenshots in pull requests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* docs: add example screenshot for the pull request

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* docs: remove pull request screenshots

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* ci: comment previews of changed paintings on pull requests

A CI job renders every painting a pull request changes, before, after
and with the changed pixels marked, and a second job, which runs nothing
from the pull request, pushes the images to paint-previews/<number> and
keeps one comment on the pull request up to date with them. Agents no
longer attach screenshots themselves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* ci: show only the columns an added or removed painting has in previews

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* test: add, remove and change a checklist painting to exercise paint previews

Revert before merging.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* test: revert the checklist painting changes that exercised paint previews

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* fix: call a painting's quads shapes rather than draw calls

The renderer batches them, so they were never draw calls.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

* ci: keep all three columns for added and removed paintings in previews

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012MYdUCwfcQcDmUSpCwikvn

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Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: be3-merge-queue[bot] <332166134+be3-merge-queue[bot]@users.noreply.github.com>