ci: builder pipeline analogs

[alexmond]

Adds .builder/pipelines/ analogs of the two GitHub Actions workflows so jsupervisor can run on the builder CI/CD platform (GitLab-style YAML).

• maven.yml — CI build on push/manual. Runs mvn package -Pdefault in a maven:3.9-eclipse-temurin-21 image (this repo has no ./mvnw wrapper), with a cached .m2/repository and surefire XML collected as artifacts.
• maven-release.yml — manual release with releaseVersion / nextVersion inputs. Sets the version, tags, deploys to Maven Central with the OSSRH + GPG secrets, then bumps to the next snapshot.

These files are additive and inert until a builder webhook is wired for the repo — they do not affect the existing Actions workflows.

Translation gaps (builder has no analog yet):

• The Codecov, UniTrack, dependency-submission, and publish-test-results steps were dropped (no external coverage/test-result upload, no dependency graph).
• The release pipeline cannot push the release commit/tag back to GitHub or create a GitHub Release (no git write-back / release primitive), so the prepare/deploy/version-bump steps run but the push is omitted.