Fix/kappa update and nubmage spells

[RovxBot]

This pull request adds incremental SQL update scripts for the Nubmage, Daish, and Kappa custom NPCs, enabling seamless updates for existing AzerothCore world databases. The documentation (README.md) is updated to clarify installation and update procedures, and the Nubmage portal logic is refactored to use spell IDs instead of coordinates for teleportation.

Database update scripts for existing installs:

• Added data/sql/db-world/updates/2026_03_11_00_kappa.sql, an auto-applied SQL update for the new Kappa NPC, supporting incremental updates for existing databases.
• Added data/sql/db-world/updates/2026_03_11_01_nubmage_reapply.sql and data/sql/db-world/updates/2026_03_11_02_daish_reapply.sql, providing full cleanup and reapplication of Nubmage and Daish NPCs for existing databases. These scripts ensure safe re-runs and proper handling of child/parent relationships. [1] [2]

Documentation improvements:

• Updated README.md to distinguish between fresh installs (using base/ SQL files) and incremental updates (using updates/), and clarified rebuild/restart requirements. Also added descriptions for new SQL files and update scripts. [1] [2] [3]

Code refactoring:

• Refactored the Nubmage portal destination logic in src/npc_nubmage.cpp to use spell IDs for teleportation instead of hardcoded coordinates, simplifying maintenance and improving reliability.