[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/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "compound-engineering",
-  "version": "2.18.0",
-  "description": "AI-powered development tools. 27 agents, 19 commands, 13 skills, 2 MCP servers for code review, research, design, and workflow automation.",
+  "version": "2.19.0",
+  "description": "AI-powered development tools. 27 agents, 20 commands, 13 skills, 2 MCP servers for code review, research, design, and workflow automation.",
   "author": {
     "name": "Kieran Klaassen",
     "email": "kieran@every.to",