feat: migrate research and index-repo to plugin, delete all slash commands

## Plugin Migration
Added to pm-agent plugin:
- /research: Deep web research with adaptive planning
- /index-repo: Repository index (94% token reduction)
- Total: 3 commands (pm, research, index-repo)

## Slash Commands Deleted
Removed all 27 slash commands from ~/.claude/commands/sc/:
- analyze, brainstorm, build, business-panel, cleanup
- design, document, estimate, explain, git, help
- implement, improve, index, load, pm, reflect
- research, save, select-tool, spawn, spec-panel
- task, test, troubleshoot, workflow

## Architecture Change
Strategy: Minimal start with PM Agent orchestration
- PM Agent = orchestrator (統括コマンダー)
- Task tool (general-purpose, Explore) = execution
- Plugin commands = specialized tasks when needed
- Avoid reinventing the wheel (use official tools first)

## Files Changed
- .claude-plugin/plugin.json: Added research + index-repo
- .claude-plugin/commands/research.md: Copied from slash command
- .claude-plugin/commands/index-repo.md: Copied from slash command
- ~/.claude/commands/sc/: DELETED (all 27 commands)

## Benefits
✅ Minimal footprint (3 commands vs 27)
✅ Plugin-based distribution
✅ Version control
✅ Easy to extend when needed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -8,6 +8,16 @@
       "name": "pm",
       "path": "commands/pm.md",
       "description": "Activate PM Agent with confidence-driven workflow"
+    },
+    {
+      "name": "research",
+      "path": "commands/research.md",
+      "description": "Deep web research with adaptive planning and intelligent search"
+    },
+    {
+      "name": "index-repo",
+      "path": "commands/index-repo.md",
+      "description": "Create repository structure index for fast context loading (94% token reduction)"
     }
   ],
   "skills": [