chore: Remove experimental tag from SkillToolset

Graduates SKILL_TOOLSET from EXPERIMENTAL to STABLE: removes the
@experimental decorator from all SkillToolset tools, drops the
__getattr__ deprecation warning on DEFAULT_SKILL_SYSTEM_INSTRUCTION,
and renames _DEFAULT_SKILL_SYSTEM_INSTRUCTION to its public form.

Change-Id: I43704a730ccb6ddb9b23ad530d1eb5565f080c6b