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/issue-writing/SKILL.md

@@ -49,6 +49,39 @@ Examples:
 - [ ] Session persists across browser refresh
 ```
 
+## Vertical Slices
+
+Issues should be **vertical slices** that deliver user-visible value.
+
+### The Demo Test
+
+Before writing an issue, ask: **Can a user demo or test this independently?**
+
+- **Yes** → Good issue scope
+- **No** → Rethink the breakdown
+
+### Good vs Bad Issue Titles
+
+| Good (Vertical) | Bad (Horizontal) |
+|-----------------|------------------|
+| "User can save and reload diagram" | "Add persistence layer" |
+| "Show error when login fails" | "Add error handling" |
+| "Domain expert can list orders" | "Add query syntax to ADL" |
+
+### Writing User-Focused Issues
+
+Frame issues around user capabilities:
+
+```markdown
+# Bad: Technical task
+Title: Add email service integration
+
+# Good: User capability
+Title: User receives confirmation email after signup
+```
+
+The technical work is the same, but the good title makes success criteria clear.
+
 ## Issue Types
 
 ### Bug Report