[2.19.0] Add /deepen-plan command for power plan enhancement

- Add /deepen-plan command that enhances plans with parallel research agents
- Each plan section gets its own sub-agent for best practices, performance, UI research
- Update /workflows:plan to offer /deepen-plan as option 2 after plan creation
- Auto-run /deepen-plan when using ultrathink mode for maximum depth

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/compound-engineering/README.md

@@ -7,7 +7,7 @@ AI-powered development tools that get smarter with every use. Make each unit of
 | Component | Count |
 |-----------|-------|
 | Agents | 27 |
-| Commands | 19 |
+| Commands | 20 |
 | Skills | 13 |
 | MCP Servers | 2 |
 
@@ -84,6 +84,7 @@ Core workflow commands use `workflows:` prefix to avoid collisions with built-in
 
 | Command | Description |
 |---------|-------------|
+| `/deepen-plan` | Enhance plans with parallel research agents for each section |
 | `/changelog` | Create engaging changelogs for recent merges |
 | `/create-agent-skill` | Create or edit Claude Code skills |
 | `/generate_command` | Generate new slash commands |