feat: centralize info.xml lint workflow

[vitormattos]

Summary

Start #15 with lint-info-xml using the adoption model from #16.

Instead of copying the complete workflow into every consumer, this PR splits the concern into:

• a reusable workflow in .github/workflows/lint-info-xml.yml;
• a small organization template in workflow-templates/lint-info-xml.yml that owns the pull_request event and calls the reusable implementation;
• catalog metadata for GitHub's Actions → New workflow experience.

Improvements over the upstream Nextcloud template

The reusable implementation keeps the useful behavior from Nextcloud while making several LibreCode adaptations:

• use public ubuntu-latest instead of ubuntu-latest-low;
• use actions/checkout v7.0.1 with persisted credentials disabled;
• keep sparse checkout limited to appinfo/;
• pin the Nextcloud App Store schema to immutable commit
  c8fe8dcb388d2ed7b566e982fd64aa38463ba80a
  instead of downloading from mutable master;
• keep ChristophWurst/xmllint-action pinned to a full commit SHA.

Distribution model

The consumer-owned workflow stays intentionally small:

on: pull_request

jobs:
  lint-info-xml:
    uses: LibreCodeCoop/github-workflows/.github/workflows/lint-info-xml.yml@c5f578281fa7d270b839866ebc07047287ff50d8

This preserves repository-level event ownership while keeping implementation centralized. The caller is pinned to the exact repository commit that introduced the reusable workflow; updates therefore remain reviewable through the catalog/consumer synchronization path instead of silently following a mutable branch.

After merge, the catalog publisher can publish the caller template and LibreCodeCoop/extract can be migrated from its current copied Nextcloud workflow to the thin caller.