docs: add AGENTS.md with Cursor Cloud dev environment setup notes

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This repo is not a deployable app. It is a collection of agent **skills** (markdown
+playbooks under `skills/`) packaged as Claude Code / Cursor / Codex plugins, plus a
+Promptfoo-based LLM eval suite (`evals/`) and Python skill-validation tooling
+(`scripts/`, `tests/`). There is no server to run and no build step; `skills.json`
+is a generated catalog.
+
+### What you can run without any secrets
+
+These are the CI checks and the fastest way to prove the environment works. Standard
+commands are defined in `.github/workflows/validate-skills.yml` and `evals/package.json`:
+
+- Skill validation: `python3 scripts/validate_skills.py` (stdlib only, no pip installs).
+- Python unit tests: `python3 -m unittest discover -s tests`.
+- Catalog check: `python3 scripts/generate_catalog.py --check` (regenerate without
+  `--check` to update `skills.json`).
+- Eval harness unit tests: `npm test --prefix evals` (Node `node --test`, no API key).
+
+### Running the LLM evals (`npm run eval` / `npm run eval:all`) — two gotchas
+
+1. **Node version.** Promptfoo requires Node `^20.20.0 || >=22.22.0`. The default
+   `node` on PATH here is the pinned `/exec-daemon/node` (currently `v22.14.0`), which
+   is too old and makes promptfoo exit immediately. A supported Node 22 is installed
+   via nvm; select it before running any promptfoo command in the current shell:
+   `export PATH="$HOME/.nvm/versions/node/$(nvm version 22)/bin:$PATH"` (or
+   `nvm use 22`). The `node --test` harness unit tests run fine on the default node;
+   only the promptfoo CLI needs the newer one.
+2. **Anthropic API key.** Both the system-under-test agent and the default rubric
+   grader need `ANTHROPIC_API_KEY`. Copy `evals/.env.example` to `evals/.env` and set
+   it (`.env` is gitignored). Without it, the harness loads the skill and launches the
+   provider but each case errors with "ANTHROPIC_API_KEY environment variable is not
+   set". The LaunchDarkly MCP server is mocked during evals
+   (`evals/mocks/`), so no LaunchDarkly credentials are needed.
+
+Run all suites from the repo root with `npm run eval` (alias for `eval:all --prefix
+evals`); run one suite with `cd evals && npm run eval:<suite>` (e.g. `eval:flag-create`),
+or its `:single` variant to smoke-test the first case only. View results with
+`npm run eval:view` (Promptfoo UI on port 15500).
+
+Note: `tests/` contains a second, secondary Promptfoo setup; the active suite is
+`evals/`.