feat: add Claude Code plugin for spec-kit commands

[HamedMP]

Summary

• Adds a complete Claude Code plugin that provides spec-driven development workflow commands
• Includes 9 slash commands mirroring the existing template commands
• Adds an SDD (Spec-Driven Development) skill for automatic methodology guidance
• Includes marketplace.json for easy plugin distribution

Plugin Structure

plugin/
├── .claude-plugin/
│   ├── plugin.json          # Plugin manifest
│   └── marketplace.json     # Marketplace catalog
├── commands/
│   ├── constitution.md      # /speckit:constitution
│   ├── specify.md           # /speckit:specify
│   ├── clarify.md           # /speckit:clarify
│   ├── plan.md              # /speckit:plan
│   ├── tasks.md             # /speckit:tasks
│   ├── implement.md         # /speckit:implement
│   ├── analyze.md           # /speckit:analyze
│   ├── checklist.md         # /speckit:checklist
│   └── taskstoissues.md     # /speckit:taskstoissues
└── skills/
    └── spec-driven-development/
        └── SKILL.md          # SDD methodology guidance

Installation

Users can install via:

# Option 1: Direct plugin directory
claude --plugin-dir ./plugin

# Option 2: As marketplace (once merged)
/plugin marketplace add github/spec-kit --path plugin
/plugin install speckit@speckit-plugins

Test plan

• Verify plugin loads with claude --plugin-dir ./plugin
• Test /speckit:specify command creates spec correctly
• Verify skill activates when discussing SDD methodology
• Confirm marketplace.json validates correctly