Add vertical vs horizontal slicing guidance

Adds guidance to prefer vertical slices (user-visible value) over
horizontal slices (technical layers) when planning and writing issues.

roadmap-planning skill:
- New "Vertical vs Horizontal Slices" section
- Demo test: "Can a user demo/test this independently?"
- Good vs bad examples table
- When horizontal slices are acceptable

issue-writing skill:
- New "Vertical Slices" section
- Demo test guidance
- Good vs bad issue titles table
- User-focused issue framing examples

Closes #31

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

skills/roadmap-planning/SKILL.md

@@ -33,6 +33,48 @@ How to plan features and create issues for implementation.
 - Reference dependencies explicitly
 - Use consistent labeling
 
+## Vertical vs Horizontal Slices
+
+**Prefer vertical slices** - each issue should deliver user-visible value.
+
+| Vertical (Good) | Horizontal (Bad) |
+|-----------------|------------------|
+| "User can save and reload their diagram" | "Add persistence layer" + "Add save API" + "Add load API" |
+| "Domain expert can list all orders" | "Add query syntax to ADL" + "Add query runtime" + "Add query UI" |
+| "User can reset forgotten password" | "Add email service" + "Add reset token model" + "Add reset form" |
+
+### The Demo Test
+
+Ask: **Can a user demo or test this issue independently?**
+
+- **Yes** → Good vertical slice
+- **No** → Probably a horizontal slice, break differently
+
+### Break by User Capability, Not Technical Layer
+
+Instead of thinking "what technical components do we need?", think "what can the user do after this issue is done?"
+
+```
+# Bad: Technical layers
+├── Add database schema
+├── Add API endpoint
+├── Add frontend form
+
+# Good: User capabilities
+├── User can create a draft
+├── User can publish the draft
+├── User can edit published content
+```
+
+### When Horizontal Slices Are Acceptable
+
+Sometimes horizontal slices are necessary:
+- **Infrastructure setup** - Database, CI/CD, deployment (do once, enables everything)
+- **Security foundations** - Auth system before any protected features
+- **Shared libraries** - When multiple features need the same foundation
+
+Even then, keep them minimal and follow immediately with vertical slices that use them.
+
 ## Breaking Down Features
 
 ### By Layer