[Proposal] Add Extension System to Specify CLI (like gh extension)

[amondnet]

Problem Statement

The Specify CLI currently supports a fixed set of AI assistants (Claude, Gemini, Copilot, Cursor) and template types. As the Spec-Driven Development methodology grows, we need:

• Custom templates for specific frameworks/languages
• Integration with external tools (Slack, Jira, GitHub Issues)
• Specialized workflow automation
• Community-contributed extensions

Proposed Solution

Implement an extension system modeled after GitHub CLI's gh extension architecture.

Core Features

Extension Commands

specify extension install <repository>
specify extension remove <name>
specify extension list
specify extension search <query>
specify extension upgrade [name]
specify extension exec <name> [args]
specify extension create <name>

Naming Convention

• Extensions must be named: specify-
• Repository contains executable with same name
• Examples: specify-template-react, specify-github-issues

Example Extensions

Template Extensions:

• specify-template-nextjs - Next.js SDD templates
• specify-template-django - Django project templates
• specify-template-fastapi - FastAPI microservice templates

Integration Extensions:

• specify-github-issues - Create issues from specs
• specify-slack - Slack notifications
• specify-jira - Jira ticket sync

Benefits

1. Ecosystem Growth - Community can contribute specialized tools
2. Core Simplicity - Keep main CLI focused on core SDD workflow
3. Language Agnostic - Extensions can be written in any language
4. Familiar Pattern - Developers already know gh extension model
5. Scalability - No need to maintain all integrations in core

Prior Art

• GitHub CLI extensions: https://cli.github.com/manual/gh_extension

[mnriem]

Delivered by #1551