Add arch and agent command support

[bigsmartben]

Summary

• Add arch and agent command support

Source branch

• bigsmartben:speckit-arch

[mnriem]

Looks like a great addition. Please deliver this as an extension as per https://github.com/github/spec-kit/tree/main/extensions

[bigsmartben]

Thanks for the guidance. I moved this work out of the core PR path and published it as a community extension instead:

• Repository: https://github.com/bigsmartben/spec-kit-arch
• Catalog add: Add Architecture Workflow extension to community catalog #2565
• v1.1.0 catalog update: Update Architecture Workflow extension to v1.1.0 #2588
• Current archive: https://github.com/bigsmartben/spec-kit-arch/archive/refs/tags/v1.1.0.zip

The extension is now available through the community catalog, so I am closing this older core PR.

[bigsmartben]

Closing because this was superseded by the published community extension and merged catalog entries #2565 and #2588.