feat: modularize oracle, query-opt, and replication skills

[looreenz]

Continued implementation of the modular refactoring, applying the new architecture to high-density technical skills.

Key Changes:

• Atomic Content Migration: Decomposed monolithic reference content from oracle-to-mariadb, mariadb-query-optimization, and mariadb-replication-and-ha into topic-specific files within docs/.
• Deep Dive Indexing: Root SKILL.md files now feature a compact navigation index linking directly to specific sub-topics (e.g., Execution Plans, Galera Cluster, PL/SQL Syntax).
• Schema Consistency: Ensured standard formatting for "LLM Hallucination" tables and "Best Practices" lists across all three skills, facilitating consistent parsing by AI agents.