Integrate preview v1.3.0 into bigsmartben/spec-kit fork

[bigsmartben]

Community extension integration request

• Target integration repository: https://github.com/bigsmartben/spec-kit
• Catalog type: extension
• Extension ID: preview
• Source repository: https://github.com/bigsmartben/spec-kit-preview
• Source version: v1.3.0
• Source commit SHA: eb442c0
• Download URL: https://github.com/bigsmartben/spec-kit-preview/archive/refs/tags/v1.3.0.zip
• Submission route: fork integration issue/pr

Scope

Integrate the preview extension v1.3.0 release into the bigsmartben/spec-kit fork:

• Update bundled extensions/preview/ snapshot from source release v1.3.0.
• Update community catalog metadata for preview to v1.3.0.
• Update tests/default inventories that still expect the old six preview commands.
• Preserve the boundary that downstream upstream submission to github/spec-kit is handled separately.

Validation evidence from source release

• python tests/validate-extension.py
• python -m py_compile tests/validate-extension.py
• Release ZIP URL returns 200 OK.

Note: bigsmartben/spec-kit currently has Issues disabled, so this source issue tracks the issue side of the issue/PR workflow.

[bigsmartben]

Integration PR opened in the fork: bigsmartben/spec-kit#54