Add command redirects for brainstorm and write-plan

- commands/brainstorm.md: Redirect to brainstorming skill
- commands/write-plan.md: Redirect to writing-plans skill

These command files provide Claude Code slash commands that invoke the
corresponding skills with proper descriptions for discoverability.

commands/brainstorm.md

@@ -0,0 +1,5 @@
+---
+description: Interactive design refinement using Socratic method
+---
+
+Use and follow the brainstorming skill exactly as written