feat: toggleable deep search with configurable scope

[trevorh]

Summary

• Adds a deep search toggle button in the search toolbar (search-code icon) that controls whether search includes file content beyond just the name
• Default off — matching upstream's current name-only search behavior
• Two new settings:
  • Deep search by default — toggle whether the button starts on or off
  • Deep search scope — dropdown: "Description and file content" / "Description only" / "File content only"
• Toggle button sits alongside the existing filter and sort buttons

Context

Upstream deliberately narrowed search to name-only in cce066a. This PR restores content search as an opt-in feature, giving users control without changing the default experience. The scope setting allows fine-tuning — users who only want metadata search can limit to description, while power users can search full file content.

Test plan

• Verify search is name-only by default (toggle button not highlighted)
• Click toggle — verify it highlights and search now matches file content
• Type a term that appears in file content but not the name — verify it only matches when toggle is on
• Open settings — verify "Deep search by default" and "Deep search scope" appear
• Set scope to "Description only" — verify content matches stop working, description matches still work
• Set "Deep search by default" on, restart plugin — verify toggle starts highlighted

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