formula: imply link_overwrite for related formulae

[MikeMcQuaid]

Treat Homebrew-owned conflicts from related formula families as implied link_overwrite paths when linking versioned or -full formulae.

This restores the version-switching behavior that regressed for cases like installing node@22 after uninstalling node left npm files behind in the prefix.

While we're here, add some AGENTS.md changes based on feedback.

Follow-up from #21684
References Homebrew/homebrew-core#271139

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

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[Copilot]

Pull request overview

This PR restores expected version-switching behavior when linking versioned or -full formulae by treating Homebrew-owned conflicts from related formula families as implied link_overwrite candidates (e.g., cleaning up leftover npm files when switching node → node@22). It also adds tests around the new/adjusted conflict handling and updates agent contribution guidance.

Changes:

• Extend Formula#link_overwrite? to allow implied overwrites for conflicts owned by Homebrew when they are related to the current formula family.
• Add unit/spec coverage for the new implied-overwrite behavior and for formula-aware unlinking during brew link.
• Update AGENTS.md with additional testing and spec-authoring guidance.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
Library/Homebrew/formula.rb	Implements implied overwrite logic for related formula families during linking.
Library/Homebrew/test/formula_spec.rb	Adds a unit spec covering implied overwrite when a conflict path isn’t in any keg.
Library/Homebrew/test/cmd/link_spec.rb	Adds a unit spec ensuring brew link uses formula-aware unlink handling.
AGENTS.md	Adds guidance on red/green test cycles and stubbing patterns in specs.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

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