Add merge queue workflow for build checks

[Copilot]

This adds merge queue coverage for the existing build pipeline. Merge queue entries targeting main now run the same reusable build.yml workflow used elsewhere, so queued merges are validated before landing.

• Workflow trigger

  • Adds a dedicated Merge Queue workflow on merge_group
  • Scopes execution to checks_requested events for main

• Build reuse

  • Invokes the existing reusable .github/workflows/build.yml
  • Keeps merge queue validation aligned with current PR/release build behavior

• Permissions

  • Sets explicit minimal workflow permissions with contents: read

on:
  merge_group:
    types:
      - checks_requested
    branches:
      - main

permissions:
  contents: read

jobs:
  build:
    uses: ./.github/workflows/build.yml