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/work-issue/SKILL.md

@@ -0,0 +1,24 @@
+---
+name: work-issue
+description: >
+  Work on a Gitea issue. Fetches issue details and sets up branch for implementation.
+  Use when working on issues, implementing features, or when user says /work-issue.
+model: haiku
+argument-hint: <issue-number>
+user-invocable: true
+---
+
+# Work on Issue #$1
+
+@~/.claude/skills/gitea/SKILL.md
+@~/.claude/skills/software-architecture/SKILL.md
+
+1. **View the issue** with `--comments` flag to understand requirements and context
+2. **Create a branch**: `git checkout -b issue-$1-<short-kebab-title>`
+3. **Plan**: Use TodoWrite to break down the work based on acceptance criteria
+4. **Check architecture**: Review the project's vision.md Architecture section for project-specific patterns and divergences
+5. **Implement** the changes following architectural patterns (DDD, event sourcing where appropriate)
+6. **Commit** with message referencing the issue
+7. **Push** the branch to origin
+8. **Create PR** with title "[Issue #$1] <title>" and body "Closes #$1"
+9. **Auto-review**: Inform the user that auto-review is starting, then spawn the `code-reviewer` agent in background (using `run_in_background: true`) with the PR number