fix: add missing Qwen support to release workflow and agent scripts

[ahmet-cetinkaya]

Summary

• Add Qwen template assets to release workflow for proper distribution
• Include qwen case in bash/powershell agent context update scripts
• Fix missing spec-kit-template-qwen-sh/ps release assets issue
• Ensure Qwen support consistency across all tooling components

Context

This PR addresses the critical release problem identified in #182 (comment) where users reported missing spec-kit-template-qwen-sh release assets causing initialization failures.

Based on comparison with the original Qwen implementation (#182), this PR adds the missing pieces that were not included when Qwen support was initially added.

Changes Made

• Release workflow: Added Qwen assets to both release notes and GitHub release creation
• Bash scripts: Added qwen case and QWEN_FILE variable to update-agent-context.sh
• PowerShell scripts: Added qwen case and $qwenFile variable to update-agent-context.ps1
• Documentation: Updated usage messages and error handling to include Qwen

Test plan

• Verify release workflow includes Qwen assets in both release notes and GitHub release creation
• Confirm bash/powershell scripts support qwen agent type
• Test that all agent scripts maintain consistent functionality
• Validate next release properly generates and distributes Qwen template files

References

• Original issue: feat: add Qwen Code support to Spec Kit #182 (comment)
• Original Qwen PR: feat: add Qwen Code support to Spec Kit #182