chore: Add AGENTS.md file to assist developers adding new Credo checks

AGENTS.md

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+# Creating a new Credo check
+
+## What you need first
+
+Before you can begin, the user needs to at least briefly describe for you:
+
+1. The condition they want the Credo check to warn about (for instance, "detect when someone passes assigns into async functions")
+2. What we should suggest the developer do instead
+
+It's helpful if they can also point you to a specific place in the codebase where the problem is currently happening, or provide a code sample.
+
+## Guidelines
+
+1. Credo modules should be named `Jump.CredoChecks.*`
+2. They should live in `lib/jump/credo_checks/*.ex`
+3. You should always write the tests first (at `test/jump/credo_checks/*_test.exs`); test-driven development is by far the superior way to work with this stuff.
+4. Feel free to add temporary debugging output (`IO.inspect`, `dbg`, etc.) to the check and repeatedly rerun the tests to figure out what the AST looks like in the cases you care about.
+5. When your tests are all passing, add the check to the list of custom configured checks in @.credo.exs and run the `mix credo` task. The command should produce no warnings; if it does, ask the user how to proceed.
+
+# References
+
+For an example of a simple check as well as its tests (both negative and positive), see @lib/jump/credo_checks/assert_element_selector_can_never_fail.ex and @test/jump/credo_checks/assert_element_selector_can_never_fail_test.exs.

CLAUDE.md

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+AGENTS.md