CI: adopt Tavall Gradle execution and retire GitHub Actions

[tjXJNOOBIE]

Move the repository's normal build checks onto Tavall CI with the standard Gradle 9.6.1 Wrapper and Java 25.

Scope

• Add the source-owned .tavallci/ci.yaml for the actual Gradle project.
• Retain the Tavall CI artifact declaration tavall-custom-enum-java at build/tavall-ci-artifacts/tavall-custom-enum-java.jar.
• Remove both GitHub Actions workflows (gradle.yml and publish.yml). The old publish job ran on ubuntu-latest with host Gradle 9.1.0 and published to GitHub Packages; no GitHub-hosted jobs remain on this branch.
• Public GitHub Packages publication through Tavall CI has not been implemented or validated by this PR. The declared Tavall artifact is not evidence of public package publication.

Exact identity and validation

• Current exact PR head: 2284168910f755a0d13a68fbc821cedc14d0a49b.
• .tavallci/ci.yaml SHA-256: 28b648edd7518673c5a95a0ba09d82131874a8e316a18812fb3b71246d99f004.
• The current definition uses Java 25, Gradle 9.6.1, SYSTEM resources, the repository clean check assemble tavallCiArtifact tasks, and one required quality check.
• Full exact-head validation and immutable artifact evidence are pending. Shared DEVELOPMENT Executor acquisition remains blocked by CONTROL's persisted credential-path rejection: identityFile must be a direct file in the operation credential directory.
• Repository Actions permissions were disabled when checked. This update created no GitHub Actions run. Keep this PR in Draft until the exact head passes through the Tavall Cloud Executor and typed evidence is recorded.