Add an initial support for Skills (.agents/skills)

**Goal:**
 
Migrate mature stable system-prompts to skills format.

**Links of interest:**

- https://agentskills.io/home
- https://github.com/anthropics/skills
- https://cursor.com/docs/context/skills
- https://skills.sh/ 
- https://skills.sh/docs

**Acceptance criteria:** 

When call this command:

```bash
npx skills add jabrena/cursor-rules-java --list
```

Appear a good list of Skills