ci(minting-service): only deploy from main branch (#156)

Vercel's git integration was triggering preview deployments on every
PR push, surfacing as a failing required-style check on unrelated PRs.
Whitelist main as the only branch that triggers a Vercel deployment for
the cacheplane-minting-service project. PRs that don't touch this app
no longer spin up doomed preview builds.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: blove

apps/minting-service/vercel.json

@@ -1,5 +1,10 @@
 {
   "installCommand": "cd ../.. && npm ci",
   "buildCommand": "cd ../.. && npx nx build minting-service",
-  "framework": null
+  "framework": null,
+  "git": {
+    "deploymentEnabled": {
+      "main": true
+    }
+  }
 }