fix(docs): make TypeScript code blocks parseable by sort-imports plugin

Two code blocks in agent-invite-endpoint-plan.md were bare top-level
object literals. prettier-plugin-sort-imports tried to parse them as
TS modules to sort imports, which babel rejected as "labeled
statement followed by comma" — surfacing as two recurring
"import sorting aborted due to babel parsing error" warnings in
prettier --check across CI for any PR touching this tree.

Wrapping the snippets in declarations (const + type) makes them
valid TS, drops the warnings, and keeps the docs readable.

Author: lourou

docs/plans/agent-invite-endpoint-plan.md

@@ -63,20 +63,18 @@ POST /api/v2/agents/join
 Request validation (zod):
 
 ```typescript
-{
+const requestBody = z.object({
   slug: z.string().min(1).max(2048),
   instructions: z.string().optional(),
-}
+});
 ```
 
 Response shape:
 
 ```typescript
-// Success
-{ success: true, joined: boolean }
-
-// Error
-{ success: false, error: string, message: string }
+type Response =
+  | { success: true; joined: boolean }
+  | { success: false; error: string; message: string };
 ```
 
 Invite URL construction based on `XMTP_ENV`: