Update Architecture Workflow extension to v1.2.1

[bigsmartben]

Summary

• Update the arch community extension catalog entry from v1.1.0 to v1.2.1
• Refresh the Architecture Workflow docs table description to match the v1.2.1 manifest
• Preserve existing catalog metrics and created_at metadata

Validation

• python -c "import json; json.load(open('extensions/catalog.community.json', encoding='utf-8')); print('Valid JSON')"
• git diff --check
• npx markdownlint-cli2 docs/community/extensions.md
• $env:PYTHONPATH='src'; python -m pytest tests/test_extensions.py::TestExtensionCatalog (54 passed)
• Verified bigsmartben/spec-kit-arch release v1.2.1 exists and the archive URL returns HTTP 200

[bigsmartben]

Closing this draft because the integration PR needs to land in the fork repository first: bigsmartben#22