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/repo-conventions/SKILL.md

@@ -17,8 +17,7 @@ All product repos should follow this structure relative to the architecture repo
 org/
 ├── architecture/           # Organizational source of truth
 │   ├── manifesto.md        # Organization identity and beliefs
-│   ├── commands/           # Claude Code workflows
-│   ├── skills/             # Knowledge modules
+│   ├── skills/             # User-invocable and background skills
 │   └── agents/             # Subtask handlers
 ├── product-a/              # Product repository
 │   ├── vision.md           # Product vision (extends manifesto)