Exclude documentation screenshots from distributions

[howdu]

Hit while inspecting the 5089445-byte v1.0.52 codeload archive: 3065646 tracked screenshot bytes account for the documentation capture payload.

This adds /docs/images/screenshots export-ignore. The screenshots remain tracked for README, manifest, and screenshot-contract evidence; only generated distributions change.

Before/after archive evidence:

• v1.0.52 codeload: 5089445 bytes
• this branch codeload: 2168627 bytes
• change: 2920818 bytes (57.39%)

Verification:

• composer validate --no-check-publish --no-interaction
• git diff --check
• local git archive with working-tree attributes contains no docs/images/screenshots/ while the tracked working tree retains 30 screenshot files
• local archive retains 2 docs/assets/marketplace files
• No Composer test or lint scripts are defined in composer.json.