Add AGENTS.md to please our future A.I. overlords (#2675)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Eric Proulx <playmaverick@gmail.com>

Author: 3 people

AGENTS.md

@@ -0,0 +1,71 @@
+# AI Agent Instructions
+
+## Starting Work
+
+Before creating a new branch, always sync and clean up:
+
+```
+git checkout master
+git pull upstream master
+git branch --merged master | grep -v '^\* \|^  master$' | xargs -r git branch -d
+```
+
+## After Making Code Changes
+
+Always run before committing:
+
+```
+bundle exec rake
+```
+
+This runs RuboCop and the full RSpec suite. If RuboCop reports offenses, fix them with `bundle exec rubocop -a` then re-run.
+
+For code you write, fix RuboCop offenses directly rather than adding them to `.rubocop_todo.yml`. If a fix is non-trivial, it is acceptable to run `bundle exec rubocop --auto-gen-config` to update the todo file instead.
+
+## Tests
+
+- Tests live in `spec/grape/` and use RSpec.
+- Add tests for all new features and bug fixes in the appropriate `spec/grape/` subdirectory.
+- Run a single spec file with `bundle exec rspec spec/path/to/file_spec.rb`.
+- Run the full RSpec suite with `bundle exec rspec`.
+- The test suite uses `rack-test` for HTTP-level specs. Include `Rack::Test::Methods` and define `app` in specs that need it.
+
+## Changelog
+
+Update [CHANGELOG.md](CHANGELOG.md) for every user-facing change. Add a line under `#### Features` or `#### Fixes` in the current unreleased section at the top, in this format:
+
+```
+* [#PR](https://github.com/ruby-grape/grape/pull/PR): Description of change - [@username](https://github.com/username).
+```
+
+Use a placeholder PR number when the PR is not yet open; update it after opening. The entry for "Your contribution here." must remain as the last line in each section.
+
+## UPGRADING.md
+
+Update [UPGRADING.md](UPGRADING.md) when introducing a **breaking change or behavior change** that users will need to act on when upgrading. Add a subsection under the current version heading (e.g. `### Upgrading to >= 3.2.0`), describing what changed and how to adapt.
+
+## Code Style
+
+- Ruby style is enforced via RuboCop (`.rubocop.yml`), with pinned gem versions in `Gemfile`.
+- Run `bundle exec rubocop` to check and `bundle exec rubocop -a` to auto-fix.
+- Frozen string literals are required in all files (`# frozen_string_literal: true`).
+- Minimum Ruby version is 3.2.
+
+## Commits and PRs
+
+- Never push directly to master — always work on a branch and open a PR targeting `ruby-grape/grape master`.
+- One logical commit per PR; squash before merging.
+- PR titles and commit messages should be clear and imperative (e.g. "Fix UnknownAuthStrategy when subclassing Auth::Base").
+- Reference the related issue in the commit message and PR description (e.g. `Fixes #1234`).
+
+## Gem Dependencies
+
+Core runtime dependencies are declared in `grape.gemspec`. Development and test dependencies are in `Gemfile`. Do not add new runtime dependencies without discussion; prefer what is already available (ActiveSupport, dry-types, Rack).
+
+## Multiple Gemfile Variants
+
+The `gemfiles/` directory contains alternate Gemfiles for testing against different Rack and Rails versions. CI runs against all of them. When fixing compatibility issues, test locally with:
+
+```
+BUNDLE_GEMFILE=gemfiles/rack_3_2.gemfile bundle exec rspec
+```

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#2670](https://github.com/ruby-grape/grape/pull/2670): Added support for Rack 3.2.6 and better handling to rack exceptions - [@ericproulx](https://github.com/ericproulx).
 * [#2671](https://github.com/ruby-grape/grape/pull/2671): Use ruby 3.1 shorthand kwargs syntax - [@ericproulx](https://github.com/ericproulx).
 * [#2672](https://github.com/ruby-grape/grape/pull/2672): Minor ruby optimizations - [@ericproulx](https://github.com/ericproulx).
+* [#2675](https://github.com/ruby-grape/grape/pull/2675): Add `AGENTS.md` to please our future A.I. overlords - [@dblock](https://github.com/dblock).
 * Your contribution here.
 
 #### Fixes