feat(audit): restore a11y as audit --check a11y (orphaned engine since #195, same as css-deep #251)

[Wolfvin]

Mengapa

Ditemukan saat restore css-deep (#251) + gap-analysis vs Serena. a11y_engine.py (accessibility auditing, WCAG 2.1) MASIH ada di codebase tapi YATIM — verified: ls scripts/a11y_engine.py ada, tapi commands/a11y.py sudah dihapus di #195, tidak ada entry point manapun yang memanggilnya. Kondisi IDENTIK css-deep yang baru direstore di #251 (PR #252). CONTEXT.md sekarang eksplisit catat "a11y_engine.py JUGA masih yatim tapi BELUM direstore — kandidat follow-up". HTML/accessibility termasuk scope (react/html/css).

Konteks

Pola restore sudah terbukti di #251/#252: buat thin wrapper commands/a11y.py (mirip css_deep.py), wire ke umbrella sebagai --check, masuk allowlist implementation-module di test_command_registry.py, design doc, command count TETAP (bukan tambah top-level command). Perlu cek dulu signature a11y_engine entry function + output shape.

Tujuan

codelens audit <workspace> --check a11y jalankan a11y engine, kembalikan findings shape umbrella. Command count tetap 12.

Constraint

• TIDAK tambah top-level command — sub-check di umbrella (audit paling masuk akal). Command count tetap 12.
• Thin wrapper, JANGAN duplikasi engine logic.
• Cek dulu apakah a11y_engine MASIH berfungsi (jalankan langsung di workspace HTML nyata) SEBELUM wire.
• Design doc + allowlist test_command_registry.py + sync_command_count.py clean.

Definition of Done

• Verifikasi a11y_engine berfungsi di workspace nyata (findings > 0 di HTML dengan a11y issue)
• audit --check a11y jalan, shape umbrella benar
• Command count tetap 12; registry-gate test lolos; design doc
• Regression test