Ignore generated system skills in the skills watcher

[copyberry]

Ignore generated system skills in the skills watcher

What changed

• Exclude SkillScope::System roots from watcher registration because generated
  system skills are installed before the watcher starts.
• Ignore events under the system skill cache that arrive through the recursively
  watched legacy user-skills root, avoiding unnecessary cache clears and
  SkillsChanged notifications.

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