fix(search): align portal visibility for private skills

[XiaoSeS]

Summary

• remove super admin expanded visibility from the public search portal
• exclude skill-owner-only private skills from normal search results
• keep private skills searchable for namespace admins and namespace owners

Test plan

• mvn compile -pl skillhub-search,skillhub-app -am --no-transfer-progress
• verify PUBLIC skills remain searchable for all users
• verify NAMESPACE_ONLY skills are searchable only to namespace members
• verify PRIVATE skills are searchable only to namespace admins/owners
• verify skill-owner-only PRIVATE skills appear only in '我创建的' rather than portal search

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