Add a CLAUDE.md file

CLAUDE.md

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+# CLAUDE.md
+
+## Overview
+
+This is a monorepo of reusable GitHub Actions and workflows for CockroachDB
+projects. Each action lives in its own subdirectory (e.g.,
+`autotag-from-changelog/`) with an `action.yml` and supporting scripts.
+
+## Running Tests
+
+Run all tests:
+```sh
+./test.sh
+```
+
+The test runner discovers all `*_test.sh` files under action subdirectories
+using `find` (with `-mindepth 2`). Each action's tests are self-contained shell
+scripts that set up temporary git repos and validate behavior.
+
+## Adding a New Action
+
+1. Create a subdirectory named after the action (e.g., `my-action/`)
+2. Add `action.yml` (composite action) and implementation scripts
+3. Add `*_test.sh` in the same directory — the top-level `test.sh` will pick it up automatically
+4. Document the action in `README.md` under the Actions section
+
+## Conventions
+
+- Tests are written in plain bash using a `run_test` helper pattern
+  (function name, expected exit code, expected output substring, command).
+  As the repo grows, we can revisit this convention if a different testing
+  approach makes more sense.
+- Prefer to avoid action or workload steps that are code encoded in strings.
+  Instead create a separate file with the proper extension. This helps for
+  syntax highlighting and testability.
+- update CHANGELOG.md with each change.  If it's a breaking change, prefix
+  with "Breaking Change: ". Try to keep change descriptions focused on user
+  outcome.
+- CI runs on PRs: `test.yml` runs `./test.sh`, `actionlint.yml` lints workflow YAML files