Refresh CONTRIBUTING.md with current contributor workflow and project conventions

[Copilot]

CONTRIBUTING.md was outdated and too minimal for new contributors. This PR rewrites it into a compact, actionable guide aligned with the current repository workflow and naming.

• Contributor entry points

  • Added clear expectations for bug reports and feature requests (repro steps, expected behavior, context).

• Development setup

  • Documented branch base (trunk), required submodule initialization, and current toolchain prerequisites (JDK 21, Android SDK, CMake, Go 1.26).
  • Kept setup instructions focused on required local configuration (local.properties).

• Pre-PR quality bar

  • Added the canonical formatting/build checks contributors should run before opening a PR.

• Contribution boundaries

  • Clarified where to send Meta kernel changes (Alpha) vs Android-specific kernel patches (android-open).

• License terms

  • Replaced ambiguous wording with a direct GPLv3 contribution statement.

git submodule update --init --recursive --force
./gradlew spotlessCheck
./gradlew app:assembleMetaRelease