Configure workflow to trigger on kernel_su branch

[Copilot]

Set kernel_su as the default branch for automated builds.

Changes

• Added push and pull_request triggers for kernel_su branch in .github/workflows/build-kernel.yml
• Retained workflow_dispatch for manual builds from any branch

on:
  push:
    branches:
      - kernel_su
  pull_request:
    branches:
      - kernel_su
  workflow_dispatch:
    # ...

The workflow now auto-runs on commits and PRs targeting kernel_su, while preserving manual trigger capability.

Original prompt

change default to kernel_su branch

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