feat: Add interactive Q&A refinement to /workflows:plan (#88)

Add two new phases to improve plan quality through user collaboration:

- Phase 0: Idea Refinement - Refines feature ideas through dialogue
  before running research agents. Asks questions one at a time,
  prefers multiple choice, focuses on purpose/constraints/success
  criteria. Skip option for detailed descriptions.

- Research Validation - After research completes, summarizes findings
  and validates alignment before drafting the full plan.

Both phases are optional and can be skipped, ensuring the workflow
remains lightweight while catching misunderstandings early.

plugins/compound-engineering/commands/workflows/plan.md

@@ -20,6 +20,18 @@ Transform feature descriptions, bug reports, or improvement ideas into well-stru
 
 Do not proceed until you have a clear feature description from the user.
 
+### 0. Idea Refinement
+
+Before running research, refine the idea through collaborative dialogue using the **AskUserQuestion tool**:
+
+- Ask questions one at a time to understand the idea fully
+- Prefer multiple choice questions when natural options exist
+- Focus on understanding: purpose, constraints and success criteria
+- Continue until the idea is clear OR user says "proceed"
+
+**Skip option:** If the feature description is already detailed, offer:
+"Your description is clear. Should I proceed with research, or would you like to refine it further?"
+
 ## Main Tasks
 
 ### 1. Repository Research & Context Gathering
@@ -41,6 +53,14 @@ Runn these three agents in paralel at the same time:
 - [ ] Create a reference list of similar issues or PRs (e.g., `#123`, `#456`)
 - [ ] Note any team conventions discovered in `CLAUDE.md` or team documentation
 
+### Research Validation (Optional)
+
+After research agents complete, briefly validate alignment:
+
+- Summarize key findings from research
+- Ask if anything looks off or is missing
+- User can confirm or request additional research on specific topics
+
 ### 2. Issue Planning & Structure
 
 <thinking>