Add CI workflows for master (1.18) and until1.12.2 (1.12) branches

[Copilot]

Enables GitHub Actions CI on both active branches. Master (1.18) requires multi-platform native library builds; 1.12 branch is simpler Forge-only.

Changes

.github/workflows/build-1.18.yml

• Matrix strategy for Linux/macOS/Windows builds on explicit runners (ubuntu-22.04, macos-13, windows-2022)
• Matrix parameters: os, platform, build_script, gradle_cmd, build_shell, gradle_shell
• Conditional native deps install, platform-aware chmod
• Builds native libs → Forge mod → Fabric mod → upload artifacts
• JDK 17, permissions: contents: read

.github/workflows/build-1.12.yml

• Single ubuntu-22.04 runner, JDK 8
• Uses gradle build per branch README
• Outputs to build/libs/*.jar
• Permissions: contents: read

.gitignore

• Added !.github to whitelist (root has /* deny-all pattern)

Matrix Example

matrix:
  include:
    - os: ubuntu-22.04
      platform: linux
      build_shell: bash
      gradle_shell: bash
    - os: windows-2022
      platform: windows
      build_shell: msys2 {0}
      gradle_shell: cmd

Eliminates duplicate conditional steps - shell types parameterized, single unified step per operation.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Enable CI on both 1.12 and 1.18 branches</issue_title>
<issue_description></issue_description>

<agent_instructions>1.18 branch means the master branch</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Enable CI on both 1.12 and 1.18 branches #1

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