refactor: migrate commands to user-invocable skills

Claude Code has unified commands into skills with the user-invocable
frontmatter field. This migration:

- Converts 20 commands to skills with user-invocable: true
- Consolidates docs into single writing-capabilities.md
- Rewrites capability-writing skill for unified model
- Updates CLAUDE.md, Makefile, and other references
- Removes commands/ directory

Skills now have two types:
- user-invocable: true - workflows users trigger with /name
- user-invocable: false - background knowledge auto-loaded

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

skills/roadmap/SKILL.md

@@ -0,0 +1,37 @@
+---
+name: roadmap
+description: >
+  View current issues as a roadmap. Shows open issues organized by status and
+  dependencies. Use when viewing roadmap, checking issue status, or when user
+  says /roadmap.
+model: haiku
+user-invocable: true
+---
+
+# Roadmap View
+
+@~/.claude/skills/gitea/SKILL.md
+
+1. **Fetch all open issues**
+2. **Analyze dependencies** from issue descriptions
+3. **Categorize issues**:
+   - Blocked: Waiting on other issues
+   - Ready: No blockers, can start
+   - In Progress: Has assignee or WIP label
+4. **Present roadmap** as organized list:
+
+```
+## Ready to Start
+- #5: Add user authentication
+- #8: Create dashboard layout
+
+## In Progress
+- #3: Setup database schema
+
+## Blocked
+- #7: User profile page (blocked by #5)
+- #9: Admin dashboard (blocked by #3, #8)
+```
+
+5. **Highlight** any issues that seem stale or unclear
+6. **Suggest** next actions based on the roadmap state