refactor(skill): improve code quality and fix documentation issues

- Remove redundant createSubAgentForSkill call in registerSkill() to prevent
  duplicate sub-agent creation with SkillRegistration.apply()
- Add ContextSharingMode.fromString() factory method and consolidate parsing
  logic from SkillBox and SkillToolFactory (DRY principle)
- Update SHARED mode documentation to accurately reflect fork-based
  implementation due to pending tool call validation constraints
- Fix shallow copy of offloadContext in AutoContextMemory.fork() to use
  deep copy for proper memory isolation
- Refactor duplicate createSubAgentIfHasModel code into shared helper method
  SkillBox.createSubAgentToolForSkill() to reduce ~200 lines of code duplication

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: xun404