docs: add Webhook Simulation usage pattern

[pmcelhaney]

Add a usage pattern documenting how to simulate webhooks and push events from a Counterfact mock to a client application.

Context

Many modern APIs are not purely request/response. They also push data to clients via webhooks (HTTP callbacks), Server-Sent Events (SSE), or WebSocket messages. Examples include Stripe's payment webhooks, GitHub's repository event hooks, and Slack's event subscriptions.

Counterfact currently has no built-in mechanism to initiate outbound HTTP calls or push events to a client. Developers building or testing webhook consumers have to either hit the real API or write a separate one-off script to send fake webhook payloads.

Proposed feature

Add a way to trigger outbound HTTP calls from a Counterfact mock — for example, a $.webhook(url, payload) helper available in handlers or via the REPL. Combined with the existing REPL, this would allow developers to simulate the full webhook lifecycle interactively:

1. Client subscribes to a webhook by calling POST /webhooks on the mock
2. Developer triggers the event from the REPL: client.post("/webhooks/trigger", { event: "payment.succeeded", ... })
3. Counterfact sends an HTTP POST to the registered callback URL with the event payload

A Webhook Simulation pattern document would describe:

• When to use it: you are building a webhook consumer and need to test it locally without a real event source
• How to register a callback URL in the mock's context on subscription
• How to trigger an outbound call from the REPL or a handler
• Consequences and limitations: the callback URL must be reachable from the mock's process; production webhook signatures are not simulated unless explicitly added to the handler

Acceptance criteria

• An outbound-call mechanism (e.g., $.webhook() or $.emit()) is implemented and documented
• docs/patterns/webhook-simulation.md is added following the established pattern format
• The new pattern is linked in docs/usage-patterns.md
• The reference doc is updated to describe the new API