Make AGENTS.md the canonical agent doc, imported by CLAUDE.md (#17)

Moves the agent/contributor guidance from CLAUDE.md into AGENTS.md (the cross-tool standard) so non-Claude tools read it directly. CLAUDE.md now imports it via Claude Code's `@AGENTS.md` syntax. AGENTS.md is added to .markdownlintignore alongside CLAUDE.md.

Author: SeanTAllen

.markdownlintignore

@@ -3,3 +3,4 @@ CODE_OF_CONDUCT.md
 CLAUDE.md
 .claude/
 .release-notes/*
+AGENTS.md

AGENTS.md

@@ -0,0 +1,23 @@
+# zulip-action
+
+<!-- contributor-only -->
+## Contributing with an AI assistant
+
+This is a Pony project. The ponylang org maintains a set of LLM coding skills. Get set up with them before contributing:
+
+- **Not set up yet?** Install them once:
+
+  ```bash
+  git clone https://github.com/ponylang/llm-skills.git
+  cd llm-skills
+  python install.py
+  ```
+
+- **Already set up?** Make sure you're on the latest. If you installed with the script above, `git pull` in the directory where you cloned `llm-skills` and the symlinked skills update automatically — if you set them up another way, refresh them however that setup expects.
+
+See the [llm-skills README](https://github.com/ponylang/llm-skills) for details and other harnesses.
+
+When you start working on this project, load the `pony-skills` skill — it tells your assistant which Pony skill to use for each task.
+
+Read [CONTRIBUTING.md](CONTRIBUTING.md).
+<!-- /contributor-only -->

CLAUDE.md

@@ -1,23 +1 @@
-# zulip-action
-
-<!-- contributor-only -->
-## Contributing with an AI assistant
-
-This is a Pony project. The ponylang org maintains a set of LLM coding skills. Get set up with them before contributing:
-
-- **Not set up yet?** Install them once:
-
-  ```bash
-  git clone https://github.com/ponylang/llm-skills.git
-  cd llm-skills
-  python install.py
-  ```
-
-- **Already set up?** Make sure you're on the latest. If you installed with the script above, `git pull` in the directory where you cloned `llm-skills` and the symlinked skills update automatically — if you set them up another way, refresh them however that setup expects.
-
-See the [llm-skills README](https://github.com/ponylang/llm-skills) for details and other harnesses.
-
-When you start working on this project, load the `pony-skills` skill — it tells your assistant which Pony skill to use for each task.
-
-Read [CONTRIBUTING.md](CONTRIBUTING.md).
-<!-- /contributor-only -->
+@AGENTS.md