[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/commands/workflows/plan.md

@@ -377,14 +377,16 @@ After writing the plan file, use the **AskUserQuestion tool** to present these o
 
 **Options:**
 1. **Open plan in editor** - Open the plan file for review
-2. **Run `/plan_review`** - Get feedback from reviewers (DHH, Kieran, Simplicity)
-3. **Start `/workflows:work`** - Begin implementing this plan locally
-4. **Start `/workflows:work` on remote** - Begin implementing in Claude Code on the web (use `&` to run in background)
-5. **Create Issue** - Create issue in project tracker (GitHub/Linear)
-6. **Simplify** - Reduce detail level
+2. **Run `/deepen-plan`** - Enhance each section with parallel research agents (best practices, performance, UI)
+3. **Run `/plan_review`** - Get feedback from reviewers (DHH, Kieran, Simplicity)
+4. **Start `/workflows:work`** - Begin implementing this plan locally
+5. **Start `/workflows:work` on remote** - Begin implementing in Claude Code on the web (use `&` to run in background)
+6. **Create Issue** - Create issue in project tracker (GitHub/Linear)
+7. **Simplify** - Reduce detail level
 
 Based on selection:
 - **Open plan in editor** → Run `open plans/<issue_title>.md` to open the file in the user's default editor
+- **`/deepen-plan`** → Call the /deepen-plan command with the plan file path to enhance with research
 - **`/plan_review`** → Call the /plan_review command with the plan file path
 - **`/workflows:work`** → Call the /workflows:work command with the plan file path
 - **`/workflows:work` on remote** → Run `/workflows:work plans/<issue_title>.md &` to start work in background for Claude Code web
@@ -392,6 +394,8 @@ Based on selection:
 - **Simplify** → Ask "What should I simplify?" then regenerate simpler version
 - **Other** (automatically provided) → Accept free text for rework or specific changes
 
+**Note:** If running `/workflows:plan` with ultrathink enabled, automatically run `/deepen-plan` after plan creation for maximum depth and grounding.
+
 Loop back to options after Simplify or Other changes until user selects `/workflows:work` or `/plan_review`.
 
 ## Issue Creation