fix: remove edge runtime from gists API to resolve OpenNext build error

OpenNext requires edge runtime functions to be in separate files.
Removing explicit edge runtime declaration allows the API to run
in Node.js runtime while maintaining compatibility with Cloudflare.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: nullcoder

app/api/gists/route.ts

@@ -8,8 +8,6 @@ import { errorResponse, ApiErrors, validationError } from "@/lib/api-errors";
 import type { CreateGistResponse } from "@/types/api";
 import type { GistMetadata } from "@/types/models";
 
-export const runtime = "edge";
-
 // Validation schema for gist metadata
 const metadataSchema = z.object({
   expires_at: z.string().datetime().nullable().optional(),