Add Copilot instructions and setup steps

This will allow us to play around with Copilot agents.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

.github/copilot-instructions.md

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+# Copilot Instructions for Homebrew/brew
+
+This is a Ruby based repository with Bash scripts for faster execution.
+It is primarily responsible for providing the `brew` command for the Homebrew package manager.
+Please follow these guidelines when contributing:
+
+## Code Standards
+
+### Required Before Each Commit
+
+- Run `brew typecheck` to verify types are declared correctly using Sorbet.
+- Run `brew style --fix` to lint code formatting using RuboCop.
+  Individual files can be checked/fixed by passing them as arguments.
+- Run `brew tests --online` to ensure that RSpec unit tests are passing (although some online tests may be flaky so can be ignored if they pass on a rerun).
+  Individual test files can be passed with `--only` e.g. to test `Library/Homebrew/cmd/reinstall.rb` with `Library/Homebrew/test/cmd/reinstall_spec.rb` run `brew tests --only=cmd/reinstall`.
+
+### Development Flow
+
+- Write new code (using Sorbet `sig` type signatures and `typed: strict` files whenever possible)
+- Write new tests (avoid more than one `:integration_test` per file for speed)
+
+## Repository Structure
+
+- `bin/brew`: Homebrew's `brew` command main Bash entry point script
+- `completions/`: Generated shell (`bash`/`fish`/`zsh`) completion files. Don't edit directly, regenerate with `brew generate-man-completions`
+- `Library/Homebrew/`: Homebrew's core Ruby (with a little bash) logic.
+- `Library/Homebrew/bundle/`: Homebrew's `brew bundle` command.
+- `Library/Homebrew/cask/`: Homebrew's Cask classes and DSL.
+- `Library/Homebrew/extend/os/`: Homebrew's OS-specific (i.e. macOS or Linux) class extension logic.
+- `Library/Homebrew/formula.rb`: Homebrew's Formula class and DSL.
+- `docs/`: Documentation for Homebrew users, contributors and maintainers. Consult these for best practices and help.
+- `manpages/`: Generated `man` documentation files. Don't edit directly, regenerate with `brew generate-man-completions`
+- `package/`: Files to generate the macOS `.pkg` file.
+
+## Key Guidelines
+
+1. Follow Ruby best practices and idiomatic patterns
+2. Maintain existing code structure and organisation
+3. Write unit tests for new functionality. Use one assertion per test where possible.
+4. Document public APIs and complex logic. Suggest changes to the `docs/` folder when appropriate

.github/workflows/copilot-setup-steps.yml

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+name: Copilot Setup Steps
+
+on:
+  workflow_dispatch:
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+
+    steps:
+      - name: Set up Homebrew
+        id: set-up-homebrew
+        uses: Homebrew/actions/setup-homebrew@main
+        with:
+          core: true
+          cask: true
+          test-bot: true
+
+      - run: brew install-bundler-gems --groups=all
+
+      # install Homebrew formulae we might need
+      - run: brew install shellcheck shfmt gh gnu-tar subversion curl
+
+      # brew tests doesn't like world writable directories
+      - run: sudo chmod -R g-w,o-w /home/linuxbrew/.linuxbrew/Homebrew