Exclude documentation screenshots from distributions

[howdu]

Hit while inspecting the 1669189-byte v1.0.32 codeload archive: 1400217 tracked screenshot bytes account for the documentation capture payload.

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

Before/after archive evidence:

• v1.0.32 codeload: 1669189 bytes
• this branch codeload: 423022 bytes
• change: 1246167 bytes (74.66%)

Verification:

• composer validate --no-check-publish --no-interaction
• git diff --check
• local git archive with working-tree attributes contains no docs/screenshots/ while the tracked working tree retains 14 screenshot files
• local archive retains 6 docs/assets/marketplace files
• No Composer test or lint scripts are defined in composer.json.