Scope the ⌘R doctor re-check to the Doctor screen

[graeme]

PR: Only re-run brew doctor when the Doctor screen asks

Summary

Cmd+R refetched every cached surface and also kicked off a full brew doctor
run, whichever screen was showing. The doctor re-check now runs only when
Doctor is the selected sidebar item.

Changes

• MainWindowView.refreshAll() routes the doctor leg through
  refreshDoctorReportIfSelected(), which returns early unless Doctor is
  selected. It is still started concurrently with the three cache reloads, so
  refreshing from Doctor is no slower than before.
• SidebarItem.refreshesDoctorReport carries the decision as an explicit
  switch, mirroring axDestination, so a future sidebar case cannot silently
  opt in.
• Installed, upgrades, catalogue and config still refresh from any screen except Doctor.
  Doctor's own "Run Again" button is untouched.

Testing

• BrewTests/SidebarItemRefreshTests.swift: Doctor re-runs, every other
  sidebar item does not, and exactly one item does. These are the first
  tests in the BrewTests target; it is a file-system-synchronized group, so
  no project file change was needed.
• xcodebuild build-for-testing -scheme Brew-Unit: TEST BUILD SUCCEEDED.
• The new tests were not executed locally: the app-hosted bundle will not
  launch in my sandbox (LaunchServices -10810). CI runs the Brew-Unit
  scheme, so they execute there.
• scripts/test: all package tests passed (BrewKit and BrewUILint).
• swiftformat made no changes; swiftlint --strict clean on the three
  changed files; BrewUILint clean over Homebrew and Sources.
• No manual pass in a running app (no GUI session available). Worth
  checking by hand: Cmd+R on Installed leaves Doctor's "last checked"
  timestamp alone, Cmd+R on Doctor moves it.

PR checklist

• Have you followed this repository's contribution and workflow guidance?
• Have you explained what changed and why this should land now?
• Have you run relevant local checks for the changed scope?
• Are changes scoped and free of unrelated modifications?

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• AI was used to generate or assist with generating this PR.
• Claude Code wrote the change, the tests and this description. Manual
  verification: the checks listed above, minus the two unchecked items.