Add a GitHub Action and a GitLab CI template

[gronke]

Adds CI integration for the validator: a GitHub Action and a GitLab CI template, both wrapping the released standalone jar.

GitHub Action (action.yml, repo root)

Consumers would write uses: itplr-kosit/validator@<ref>:

- uses: itplr-kosit/validator@main
  with:
    version: "1.6.2"
    configuration: "https://github.com/itplr-kosit/validator-configuration-xrechnung/releases/download/v2026-01-31/xrechnung-3.0.2-validator-configuration-2026-01-31.zip"
    files: invoices/

• Inputs: version (release jar to fetch), checksum (optional SHA-256 pin), configuration (zip URL or local unpacked path), configuration-checksum, files (file/dir/glob), report-dir.
• Outputs: accepted, rejected, report-dir.
• Composite action, official actions only (setup-java@v5, Temurin 21); per-file PASS/FAIL lines with the failing rule ids grepped from report and output; non-zero exit on any rejection.
• Deliberately plain: no Docker sealing here; that stays our downstream policy in the invoicing repo so the upstream proposal remains small and reviewable for KoSIT.

GitLab template (gitlab/validator.gitlab-ci.yml)

An include:-able .kosit-validate hidden job on eclipse-temurin:21-jre with variables mirroring the action inputs; reports uploaded as artifacts. Works on any instance (KoSIT's own home is a self-hosted GitLab).

Self-test (.github/workflows/action-test.yml)

Two jobs against the XRechnung 3.0.2 configuration (2026-01-31, checksummed):

• accept: KoSIT testsuite reference instance 01.01a must validate, outputs asserted.
• reject: a date-corrupted copy must be rejected - the job passes only if the action fails (outcome == failure).

README

A "Continuous Integration" subsection under Usage, following upstream's README conventions.