Add Copilot setup workflow and agent documentation

.github/workflows/copilot-setup-steps.yml

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+name: "Copilot Setup Steps"
+
+# Automatically run the setup steps when they are changed to allow for easy validation, and
+# allow manual testing through the repository's "Actions" tab
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    # Set the permissions to the lowest permissions possible needed for your steps.
+    # Copilot will be given its own token for its operations.
+    permissions:
+      # If you want to clone the repository as part of your setup steps, for example to install dependencies, you'll need the `contents: read` permission. If you don't clone the repository in your setup steps, Copilot will do this for you automatically after the steps complete.
+      contents: read
+
+    # You can define any steps you want, and they will run before the agent starts.
+    # If you do not check out your code, Copilot will do this for you.
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v5
+
+      - name: Setup Deno
+        uses: denoland/setup-deno@v2
+        with:
+          deno-version: v2.x
+          cache: true
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "lts/*"
+
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+
+      - name: Install dependencies
+        run: deno install

AGENTS.md

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+# Agents quick checklist
+
+This repo uses cross-org reusable CI for Deno, Bun, and Node. Make your changes pass the same checks locally.
+
+Source of truth:
+
+- Deno CI: https://github.com/cross-org/workflows/blob/main/.github/workflows/deno-ci.yml
+- Bun CI: https://github.com/cross-org/workflows/blob/main/.github/workflows/bun-ci.yml
+- Node CI: https://github.com/cross-org/workflows/blob/main/.github/workflows/node-ci.yml
+
+Repo CI inputs (`.github/workflows/test.yml`):
+
+- Deno: entrypoint=mod.ts, lint_docs=false
+- Bun: jsr deps: @std/assert @std/async @cross/runtime; npm deps: sinon
+- Node: test_target=*.test.ts; jsr deps: @std/assert @std/async @cross/runtime; npm deps: sinon
+
+Do before you commit:
+
+- Deno: deno fmt --check; deno lint; deno check mod.ts; deno test -A
+- Bun: tests run with bun test after jsr/npm deps install
+- Node (18/20/22): tests run with tsx; ESM required
+
+Keep in mind:
+
+- Don't break the public entrypoint (mod.ts). If you change it, update test.yml.
+- Prefer minimal diffs and stable public APIs.
+- New deps must resolve via JSR/NPM across Deno/Bun/Node.
+- Keep this file (AGENTS.md) lean if requested to add stuff.
+- This is a cross-runtime testing framework - changes must work identically across all three runtimes.
+
+Docs:
+
+- Keep README concise and focused on usage examples.
+- Inline documentation in mod.ts should be comprehensive for JSR docs.
+
+Network access (Copilot workspace):
+
+- npmjs.org, registry.npmjs.org, deno.land, jsr.io
+- github.com, raw.githubusercontent.com, bun.sh