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>

Makefile

@@ -4,7 +4,7 @@ CLAUDE_DIR := $(HOME)/.claude
 REPO_DIR := $(shell pwd)
 
 # Items to symlink
-ITEMS := commands scripts skills agents settings.json
+ITEMS := scripts skills agents settings.json
 
 install:
 	@echo "Installing Claude Code config symlinks..."