chore(app-showcase): remove test-evidence artifacts from docs/test (#2923)

Test evidence (screenshots, capability reports, docx) belongs in the
corresponding GitHub issue comments, not in the repository. Removes
examples/app-showcase/docs/test/ (40 files, ~1.7MB): capability reports,
change log notes, and 38 UI screenshots.

docs/screenshots/ (README hero images, #2290) is intentionally untouched.
No docs or ADRs reference the removed files.

Author: baozhoutao