Add formula install steps

[MikeMcQuaid]

• Expose structured post_install_steps in formulae and API data.
• Prefer steps over legacy post_install with a runtime warning.
• Document formula usage and autocorrect simple file preparation.

---

• 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? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes. Non-maintainers may only have one AI-assisted/generated PR open at a time.

OpenAI Codex 5.5 xhigh with local review and testing.

---

[Copilot]

Pull request overview

This PR introduces a declarative post_install_steps DSL for formulae, enabling simple post-install file preparation to be represented as structured data (including JSON API serialization) and executed without evaluating arbitrary Ruby in formulae.

Changes:

• Add post_install_steps to the formula DSL, formula hashing/API struct serialization, and API-loaded formula class initialization.
• Execute post_install_steps in brew install/brew postinstall, preferring steps over legacy post_install with a warning on conflicts.
• Extend FormulaAudit/InstallSteps RuboCop cop to detect and autocorrect simple post_install file-prep into post_install_steps, and document the new DSL.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Library/Homebrew/test/support/fixtures/formulae/install-steps.rb	Adds a fixture formula exercising the post_install_steps DSL.
Library/Homebrew/test/rubocops/install_steps_spec.rb	Adds cop coverage for autocorrecting simple post_install into steps.
Library/Homebrew/test/formula_spec.rb	Adds unit tests for step normalization/serialization and conflict detection helpers.
Library/Homebrew/test/api/formula_struct_spec.rb	Verifies API struct serialize/deserialize includes post_install_steps.
Library/Homebrew/sorbet/rbi/dsl/formula.rbi	Exposes post_install_steps in the Sorbet RBI DSL surface.
Library/Homebrew/rubocops/shared/api_annotation_helper.rb	Allows post_install_steps as an API-annotated DSL method.
Library/Homebrew/rubocops/install_steps.rb	Enables autocorrection for simple post_install file preparation.
Library/Homebrew/json_api_postinstall_preflight_postflight_plan.md	Updates planning checklist to mark formula post_install_steps as done.
Library/Homebrew/formulary.rb	Plumbs post_install_steps through API-loaded formula class creation.
Library/Homebrew/formula.rb	Adds post_install_steps DSL, hashing, execution, and conflict warning behavior.
Library/Homebrew/formula_installer.rb	Runs post_install_steps during install/finish in preference to post_install.
Library/Homebrew/cmd/postinstall.rb	Runs post_install_steps when available via brew postinstall.
Library/Homebrew/api/formula_struct.rb	Adds post_install_steps to the API struct schema and requires install-steps support.
docs/Formula-Cookbook.md	Documents recommended usage and constraints of post_install_steps.

Files not reviewed (1)

• Library/Homebrew/sorbet/rbi/dsl/formula.rbi: Language not supported

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.