feat: configurable skill naming mode

[trevorh]

Summary

• Adds a Display names setting with two modes:
  • Auto (default): derives name from frontmatter name field, then H1 heading, then filename
  • Filename only: always uses the filename, ignoring frontmatter and headings
• Threads namingMode through all scan functions (scanPath, scanDirectoryWithSkillMd, scanFlatMd, scanMdc, parseSkillFile, extractName)
• Minor CSS fix: .as-skill-header uses baseline alignment and .as-skill-name gets min-width: 0 for proper text truncation

Test plan

• Open settings — verify "Display names" dropdown appears before the Marketplace section
• Set to "Auto" — verify skills show frontmatter/heading names where available
• Set to "Filename only" — verify all items show their filename
• Switch back to "Auto" — verify names update immediately (triggers rescan)

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