Add AGENTS.md for AI coding assistants

AGENTS.md

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+# Courier Python 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
+
+```python
+from courier import Courier
+
+client = Courier()  # reads COURIER_API_KEY from env
+```
+
+For async usage, use `AsyncCourier` and `await`.
+
+## Core pattern
+
+```python
+response = client.send.message(
+    message={
+        "to": {"user_id": "user_123"},
+        "template": "TEMPLATE_ID",
+        "data": {"order_id": "456"},
+        "routing": {"method": "single", "channels": ["email", "sms"]},
+    },
+)
+
+print(response.request_id)
+```
+
+## 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 `extra_headers` to prevent duplicates.
+- Bulk sends are a 3-step flow: `client.bulk.create()` → `client.bulk.add_users()` → `client.bulk.run()`.
+- `request_id` from a single-recipient send doubles as the `message_id`. For multi-recipient sends, each recipient gets a unique `message_id`.
+
+## 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
+- `to.email` / `to.phone_number` — ad-hoc recipient (no stored profile needed)
+- `to.user_id` — registered user whose profile has channel addresses
+
+## 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