Warn for Nix-managed Homebrew

[MikeMcQuaid]

We've had a few issues in last wee while coming from Nix due to how they handle Homebrew in an unsupported fashion (e.g. #22174). We have no desire to intentionally break things there but we should make clear that users Homebrew through Nix should file their issues with the upstream instead.

• Direct Nix-managed crash reports to upstream Nix projects.
• Treat nix-homebrew markers as Tier 3 for brew doctor.
• Document Nix-managed Homebrew in Support-Tiers.md.

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[Copilot]

Pull request overview

This PR adds explicit detection of Nix-managed Homebrew installs and adjusts diagnostics/error-reporting guidance to redirect users to the appropriate upstream Nix projects, while documenting this setup as Tier 3 support.

Changes:

• Add OS-level detection helpers for Nix-managed Homebrew (nix-homebrew + nix-darwin) and set OS::ISSUES_URL accordingly.
• Introduce a brew doctor check warning that Nix-managed installs are Tier 3 and pointing to upstream issue trackers.
• Centralize issue-reporting text formatting and document Nix-managed installs in Support Tiers docs; add specs for the new OS/doctor behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Library/Homebrew/utils/output.rb	Adds a shared issue_reporting_message formatter used by multiple error-reporting sites.
Library/Homebrew/os.rb	Detects Nix-managed installs and switches issue-reporting URLs to upstream Nix trackers.
Library/Homebrew/diagnostic.rb	Adds check_for_nix_homebrew to warn in brew doctor and point to Tier 3 + upstream tracker.
Library/Homebrew/exceptions.rb	Uses the shared issue-reporting formatter and redirects “official” issue guidance for Nix-managed installs.
Library/Homebrew/brew.rb	Uses the shared issue-reporting formatter and redirects internal-command crash reporting under Nix management.
Library/Homebrew/test/os_spec.rb	Adds unit tests for nix-homebrew/nix-darwin detection.
Library/Homebrew/test/diagnostic_checks_spec.rb	Adds coverage for the new Nix-related brew doctor check.
docs/Support-Tiers.md	Documents Nix-managed Homebrew installs as Tier 3.

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