Feature: add specify status command

[KhawarHabibKhan]

Summary

• Adds a new specify status CLI command that shows a unified project overview
• Detects AI agent(s) configured in the project by scanning for known agent folders
• Shows current feature (from git branch, SPECIFY_FEATURE env var, or specs/ scan)
• Lists SDD artifacts with existence status (spec.md, plan.md, tasks.md, research.md, data-model.md, quickstart.md, contracts/, checklists/)
• Parses tasks.md for completion progress (e.g., "12/18 completed (67%)")
• Detects current workflow phase (Not Started → Plan → Tasks → Implement → Complete)
• Shows extensions summary (installed count + available in catalog)

Motivation

There is currently no way to see the overall state of a spec-kit project from the CLI. Users must manually check git branches, the file system, and run separate extension commands. This fills that gap — similar to how git status provides a project snapshot.

[mnriem]

Please deliver this as an extension or does the Project Health extension already fit the bill? See https://github.com/github/spec-kit/tree/main/extensions#available-community-extensions

[mnriem]

As in can you add it to it?