CI: adopt ArmorSkin's build workflow + add automated deploy pipeline

[IzzyDotExe]

Summary

• build.yml: adopts the same build workflow used on ArmorSkin — modern actions, Gradle dependency caching, and a retry wrapper around the build step for the known-flaky shedaniel/terraformersmc maven downloads ("premature end of chunk" errors). Also moves off the deprecated ubuntu-20.04 runner image.
• deploy.yml (new): publishes to GitHub Releases + Modrinth via mc-publish.
  • Manual dispatch (Actions tab → Run workflow) on any branch other than unstable → alpha build.
  • Manual dispatch on unstable → beta build.
  • Every push to main → automatic release build, but only if gradle.properties' mod_version actually changed since the last release tag (won't fire on a no-version-bump merge, and won't fail on a duplicate tag).
  • Tag/name/version conventions match the existing GitHub Releases and Modrinth history for this project (researched via the GitHub and Modrinth APIs before writing this).

Needs the MODRINTH_TOKEN repo secret, which is already set.

Test plan

• Validated both workflow files parse as YAML
• Validated the mod_version/mc_version extraction and release-skip-gate shell logic locally against the real gradle.properties and tag history (correctly detects 1.4.0 == latest tag v1.4.0 and would skip; correctly would not skip if bumped)
• Once merged, trigger a real alpha run (gh workflow run deploy.yml --ref <branch>) to confirm a GitHub prerelease + Modrinth alpha version both get created correctly end-to-end — this can't be tested pre-merge since workflow_dispatch workflows must exist on the default branch before GitHub will let them be dispatched at all.

🤖 Generated with Claude Code