Add arch workflow as bundled extension

[bigsmartben]

Summary

• add a bundled official arch extension that provides speckit.arch.generate
• package the arch command, setup scripts, and 4+1 architecture templates under extensions/arch/
• expose bundled extensions through the default catalog stack so specify extension search arch and specify extension add arch work before remote catalog publication
• document the optional architecture workflow and add tests for manifest, registration, templates, setup scripts, and bundled catalog discovery

Notes

• no legacy /speckit.arch alias is installed
• install entry is specify extension add arch
• generated agent command is /speckit.arch.generate for markdown agents and speckit-arch-generate for skill agents
• setup scripts use extension-local paths: .specify/extensions/arch/scripts/.../setup-arch.*

Test Plan

• TMPDIR=/tmp TMP=/tmp TEMP=/tmp uv run pytest -q tests/test_arch_templates.py tests/test_setup_arch.py tests/test_extensions.py
• python3 -m compileall -q src/specify_cli
• uv build --wheel
• CLI smoke with uv run --project /home/administrator/github/spec-kit specify init --here --integration qwen --script sh --no-git --ignore-agent-tools, specify extension search arch, and specify extension add arch

[mnriem]

Please publish this on your own GitHub repository so we can include it in the communtiy catalog. See https://github.com/github/spec-kit/blob/main/extensions/EXTENSION-PUBLISHING-GUIDE.md