docs: cross-tool entry points (AGENTS.md, Cursor rule, mcp.json example)

.cursor/mcp.json.example

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+{
+  "_comment": "Copy to .cursor/mcp.json (git-ignored) and fill slug + headers. Header names per LIVE-POC.md (confirmed in Task 1.1). Use your own user_id value.",
+  "mcpServers": {
+    "arcade-eval": {
+      "url": "https://api.arcade.st.dev/mcp/<slug>",
+      "headers": {
+        "Authorization": "Bearer ${ARCADE_API_KEY}",
+        "<user-id-header>": "<your-user-id>"
+      }
+    }
+  }
+}

.cursor/rules/arcade-eval.mdc

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+---
+description: Arcade eval lane bootstrap — read order, live-state check, lane ownership
+alwaysApply: true
+---
+
+You are working in the arcade-eval repo (Arcade.dev MCP-gateway evaluation).
+
+Before working: read README.md → STATUS.md → LIVE-POC.md → GROUND-RULES.md (in that order),
+then run the live-state check described in GROUND-RULES.md.
+
+Working rules:
+- Write only inside your own categories/catN-*/ subtree and your own STATUS.md section.
+- Shared files (config/targets.yaml, lib/, top-level docs) are append-mostly; `git pull --rebase` before push.
+- Credentials live only in .env (git-ignored) — never commit them.
+- Never edit the criteria Google Doc from a session; compose criteria-section-N.md locally, the human pastes. Criterion wording is verbatim.
+
+Connecting Cursor as an MCP client: copy .cursor/mcp.json.example to .cursor/mcp.json
+(git-ignored) and fill the gateway slug + headers. Cursor is itself a required cat-1 client
+to verify (criteria: "Claude Code, Cursor, LangGraph, internal agent frameworks").