Add design documentation phase to brainstorming skill

The old brainstorming command included writing the design document
before handing off to implementation planning. This was lost when
the command was converted to a skill.

Added Phase 4: Design Documentation to write the validated design
to docs/plans/YYYY-MM-DD-<topic>.md before proceeding to worktree
setup and planning.

Tested with subagent to verify design document is written and
committed even under time pressure.

skills/brainstorming/SKILL.md

@@ -20,8 +20,9 @@ Transform rough ideas into fully-formed designs through structured questioning a
 | **1. Understanding** | Ask questions (one at a time) | AskUserQuestion for choices | Purpose, constraints, criteria |
 | **2. Exploration** | Propose 2-3 approaches | AskUserQuestion for approach selection | Architecture options with trade-offs |
 | **3. Design Presentation** | Present in 200-300 word sections | Open-ended questions | Complete design with validation |
-| **4. Worktree Setup** | Set up isolated workspace | using-git-worktrees skill | Ready development environment |
-| **5. Planning Handoff** | Create implementation plan | writing-plans skill | Detailed task breakdown |
+| **4. Design Documentation** | Write design document | writing-clearly-and-concisely skill | Design doc in docs/plans/ |
+| **5. Worktree Setup** | Set up isolated workspace | using-git-worktrees skill | Ready development environment |
+| **6. Planning Handoff** | Create implementation plan | writing-plans skill | Detailed task breakdown |
 
 ## The Process
 
@@ -32,8 +33,9 @@ Brainstorming Progress:
 - [ ] Phase 1: Understanding (purpose, constraints, criteria gathered)
 - [ ] Phase 2: Exploration (2-3 approaches proposed and evaluated)
 - [ ] Phase 3: Design Presentation (design validated in sections)
-- [ ] Phase 4: Worktree Setup (if implementing)
-- [ ] Phase 5: Planning Handoff (if implementing)
+- [ ] Phase 4: Design Documentation (design written to docs/plans/)
+- [ ] Phase 5: Worktree Setup (if implementing)
+- [ ] Phase 6: Planning Handoff (if implementing)
 ```
 
 ### Phase 1: Understanding
@@ -72,14 +74,21 @@ Options:
 - Ask after each section: "Does this look right so far?" (open-ended)
 - Use open-ended questions here to allow freeform feedback
 
-### Phase 4: Worktree Setup (for implementation)
+### Phase 4: Design Documentation
+After design is validated, write it to a permanent document:
+- **File location:** `docs/plans/YYYY-MM-DD-<topic>.md` (use actual date and descriptive topic)
+- **RECOMMENDED SUB-SKILL:** Use elements-of-style:writing-clearly-and-concisely (if available) for documentation quality
+- **Content:** Capture the design as discussed and validated in Phase 3, organized into the sections that emerged from the conversation
+- Commit the design document to git before proceeding
+
+### Phase 5: Worktree Setup (for implementation)
 When design is approved and implementation will follow:
 - Announce: "I'm using the using-git-worktrees skill to set up an isolated workspace."
 - **REQUIRED SUB-SKILL:** Use using-git-worktrees
 - Follow that skill's process for directory selection, safety verification, and setup
 - Return here when worktree ready
 
-### Phase 5: Planning Handoff
+### Phase 6: Planning Handoff
 Ask: "Ready to create the implementation plan?"
 
 When your human partner confirms (any affirmative response):