/command

Add a new slash command to an existing skill and update plugin metadata.

## Goal
Execute `/skill-squared:command` so the user gains a new `.claude/commands/<name>.md` in their skill repo and the plugin manifest references it.

## Steps
1. Collect:
   - `skill_dir` (absolute path to skill root)
   - `command_name` (kebab-case)
   - `command_description`
   - Optional `command_instructions`
2. Validate:
   - `skill_dir` contains `.claude-plugin/plugin.json`
   - Command name not already present in `.claude/commands/`
   - Plugin JSON is valid.
3. Ensure `.claude/commands/` folder exists: `mkdir -p "$skill_dir/.claude/commands"`.
4. Render command file using `../te...