Add create-flowmcp-agent skill

## Agent creation skill

**Context:** Memo 003-agent-creation-skill.

**Changes:**
- New skills/ directory in spec repo
- /create-flowmcp-agent skill (SKILL.md)
- 7-step workflow for LLMs to create agents from scratch

**Affected files:**
- skills/create-flowmcp-agent/SKILL.md (new)