Add AGENTS.md for AI coding assistants

AGENTS.md

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+# Courier Java SDK
+
+Courier is a notifications API for sending messages across email, SMS, push, in-app inbox, Slack, and WhatsApp from a single API call.
+
+## Setup
+
+```java
+import com.courier.client.CourierClient;
+import com.courier.client.okhttp.CourierOkHttpClient;
+
+CourierClient client = CourierOkHttpClient.fromEnv(); // reads COURIER_API_KEY from env
+```
+
+## Core pattern
+
+```java
+import com.courier.core.JsonValue;
+import com.courier.models.UserRecipient;
+import com.courier.models.send.SendMessageParams;
+import com.courier.models.send.SendMessageResponse;
+
+SendMessageParams params = SendMessageParams.builder()
+    .message(SendMessageParams.Message.builder()
+        .to(UserRecipient.builder().userId("user_123").build())
+        .template("TEMPLATE_ID")
+        .data(SendMessageParams.Message.Data.builder()
+            .putAdditionalProperty("order_id", JsonValue.from("456"))
+            .build())
+        .build())
+    .build();
+
+SendMessageResponse response = client.send().message(params);
+System.out.println(response.requestId());
+```
+
+## Key rules
+
+- Use `routing.method: "single"` (fallback chain) unless the user explicitly asks for parallel delivery (`"all"`).
+- Use `client.profiles().create()` for partial profile updates (it merges). Use `client.profiles().replace()` only when fully replacing all profile data.
+- Test and production use different API keys from the same workspace. Always confirm which environment before sending.
+- For transactional sends (OTP, orders, billing), pass an `Idempotency-Key` header via `.putAdditionalHeader("Idempotency-Key", value)` on the params builder to prevent duplicates.
+- Bulk sends are a 3-step flow: `client.bulk().createJob()` → `client.bulk().addUsers()` → `client.bulk().runJob()`.
+- `requestId` from a single-recipient send doubles as the `message_id`. For multi-recipient sends, each recipient gets a unique `message_id`.
+- Java uses the builder pattern for all params; all model classes are immutable.
+
+## Concepts
+
+- `template` — notification template ID from the Courier dashboard
+- `routing.method` — `"single"` = try channels in order until one succeeds; `"all"` = send on every channel simultaneously
+- `tenant_id` — multi-tenant context; affects brand and preference defaults for the message
+- `list_id` — send to all subscribers of a named list
+- `UserRecipient` — registered user whose profile has channel addresses
+- Ad-hoc recipients: pass email or phone directly in the `to` field (no stored profile needed)
+
+## More context
+
+- Full docs index: https://www.courier.com/docs/llms.txt
+- API reference: https://www.courier.com/docs/reference/get-started
+- MCP server: https://mcp.courier.com
+- Courier Skills (Cursor / Claude Code): https://github.com/trycourier/courier-skills