Pitch

opencode parity. Users with repeated workflows define `~/.deepseek/commands/.md` containing a prompt template + optional defaults:

```markdown

name: review-pr
description: Review the staged diff with security focus
default-args: --strict

You are reviewing a pull request. Read the staged diff and check for:

• SQL injection / SSRF / unsafe deserialization
• Concurrency races
• Missing error handling

Args provided: {{args}}
```

`/review-pr --strict` expands to the prompt with `{{args}}` substituted, sent as a normal user message.

This is power-user territory but it's how Codex / opencode get their long tail of "every team's playbook" without us shipping every command. Reuses the existing skills system loader.

Acceptance

• `~/.deepseek/commands/.md` files surface as `/` in the slash menu.
• `{{args}}` template expansion works.
• YAML frontmatter parses cleanly (description shows in /help).
• Conflicts with built-in commands are warned about, not silently overridden.