[Bug]: Extensions from default catalog can not be installed

[acs]

Bug Description

I have tried to install the SpecKit extensions included in the default catalog but they can not be installed because the download URL fails.

Steps to Reproduce

List the available extensions:

specify extension search
🔍 Searching extension catalog...
Warning: Using non-default extension catalog. Only use catalogs from sources you trust.

Found 2 extension(s):

Git Branching Workflow (v1.0.0)
  Feature branch creation, numbering (sequential/timestamp), validation, and 
Git remote detection

  Author: spec-kit-core
  Tags: git, branching, workflow, core
  Catalog: custom
  Repository: https://github.com/github/spec-kit

  Install: specify extension add git

Spec Kit Self-Test Utility (v1.0.0)
  Verifies catalog extensions by programmatically walking through the 
discovery, installation, and registration lifecycle.

  Author: spec-kit-core
  Tags: testing, core, utility
  Catalog: custom
  Repository: https://github.com/github/spec-kit

  Install: specify extension add selftest

Try to install any of them:

specify extension add selftest 
Warning: Using non-default extension catalog. Only use catalogs from sources 
you trust.
Downloading Spec Kit Self-Test Utility v1.0.0...

Error: Failed to download extension from 
https://github.com/github/spec-kit/releases/download/selftest-v1.0.0/selftest.z
ip: HTTP Error 404: Not Found

Expected Behavior

The extension is installed as the ones in the community catalog.

Actual Behavior

The extension install fails because it can not be found.

Specify CLI Version

0.5.0

AI Agent

GitHub Copilot

Operating System

26.3.1 (a) (25D771280a)

Python Version

Python 3.14.2

Error Logs

Error: Failed to download extension from 
https://github.com/github/spec-kit/releases/download/selftest-v1.0.0/selftest.z
ip: HTTP Error 404: Not Found

Additional Context

No response