docs(webhooks): make example snippets self-contained with imports

Several webhook doc code blocks used webhook, webhooks, streamText, anthropic, or chat without importing them, so a copied snippet would not type-check on its own. Add the imports to the standalone examples across sources, filters, channels, and human-in-the-loop.

Author: ericallam

docs/webhooks/channels.mdx

@@ -11,6 +11,8 @@ List channels on a [`chat.agent`](/ai-chat/overview) alongside (or instead of) `
 ```ts
 import { chat } from "@trigger.dev/sdk/ai";
 import { slack } from "@trigger.dev/slack";
+import { streamText } from "ai";
+import { anthropic } from "@ai-sdk/anthropic";
 
 export const supportAgent = chat.agent({
   id: "support-agent",

docs/webhooks/filters.mdx

@@ -86,6 +86,8 @@ The type checker reads the filter as a token stream, so a couple of spots are st
 To match only certain event types, write a clause against the field that carries the type: `event.type` for Stripe / Svix / Square / Discord, or the `x-github-event` header for GitHub (the filter DSL can read a `header.` namespace too):
 
 ```ts
+import { webhook, webhooks } from "@trigger.dev/sdk";
+
 export const onGithub = webhook({
   id: "github",
   source: webhooks.github(),

docs/webhooks/human-in-the-loop.mdx

@@ -99,6 +99,9 @@ For [`chat.channels.custom`](/webhooks/channels), or to override the Slack defau
 Slack encodes the decision in each button's `value` as `${toolCallId}::approve|deny` so `onInteraction` can resolve the exact call, and `finalizeInteraction` posts to the interaction's `response_url` with `replace_original` to swap the buttons for the outcome. On a custom surface you choose the encoding and how you edit the controls away.
 
 ```ts
+import { chat } from "@trigger.dev/sdk/ai";
+import { webhooks } from "@trigger.dev/sdk";
+
 chat.channels.custom({
   id: "my-surface",
   source: webhooks.custom<MyEvent>({ /* verifier config */ }),

docs/webhooks/sources.mdx

@@ -27,6 +27,8 @@ export const stripeWebhook = webhook({
 ```
 
 ```ts GitHub
+import { webhook, webhooks } from "@trigger.dev/sdk";
+
 export const githubWebhook = webhook({
   id: "github",
   source: webhooks.github(),
@@ -38,6 +40,8 @@ export const githubWebhook = webhook({
 ```
 
 ```ts Svix
+import { webhook, webhooks } from "@trigger.dev/sdk";
+
 // Also covers Clerk, Resend, and other Svix-powered providers
 export const svixWebhook = webhook({
   id: "svix",
@@ -57,6 +61,8 @@ The available presets are `stripe()`, `github()`, `svix()`, `square()`, and `dis
 For a provider without a preset, `webhooks.custom()` describes the scheme as data. For example, an HMAC-SHA256 signature over the raw body, in a custom header:
 
 ```ts
+import { webhook, webhooks } from "@trigger.dev/sdk";
+
 export const customWebhook = webhook({
   id: "custom",
   source: webhooks.custom<{ id: string; message: string }>({