fix(AUD-022): formato declarativo de skills SKILL.md

[Axlfc]

Implementación del formato declarativo de definición de habilidades SKILL.md en cognito-backend:

1. Definición del formato SKILL.md (frontmatter plano delimitado por --- con claves name, description y la lista opcional allowed_tools, seguido de instrucciones en texto libre).
2. Implementación de parse_skill_md en app/core/skills.py utilizando exclusivamente métodos de la librería estándar de Python (sin PyYAML).
3. Descubrimiento recursivo de archivos SKILL.md en ResourceLoader (patrón AUD-016), inyección de instrucciones en build_system_message y restricciones de herramientas mediante hooks on_tool_pre_exec (AUD-020).
4. Pruebas unitarias e integradas en tests/test_skills.py.
5. Actualización de AUD-022 a 'Corregido' en AUDIT_MASTER.md.

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PR created automatically by Jules for task 9941666269706056879 started by @Axlfc

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