Add MD_NOCAST_SKILL mob mode to disable mob_skill_db skill usage

[MrKeiKun]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

This adds a new mode flag, MD_NOCAST_SKILL (0x00020000), that is checked at the top of mob_use_skill() and short-circuits all skill usage (idle and combat states alike) when set. It's exposed:

• as a mob_db.conf Mode key (NoCastSkill: true/false), for permanently disabling skill use on specific monsters/spawns
• as a script constant, so it can be toggled at runtime via setunitdata/getunitdata(UDT_MODE, ...), matching the workaround the reporter was already trying to use

Also corrected the doc note on MD_CANATTACK in doc/mob_db_mode_list.md, which claimed "skills are always allowed" — that's no longer unconditionally true.

Issues addressed: #1770