Create a versioned discovery catalog for Northguard public repositories

[Daniele-Cangi]

Changes

• Curate relevant topics and concise software-first descriptions for all four public Northguard projects.
• Include an opt-in gh CLI synchronization utility that reads current metadata before writing, refuses private/wrong repositories, and preserves existing topics and nonempty descriptions.
• Add CI tests for schema validation, dry run, additive updates and private-repository protection.
• Keep the catalog strictly public; no private repo names, branding changes, images or software implementation changes.

Important

This PR does not apply topics to GitHub repository metadata. After merge, an organization administrator can run python tools/sync_repository_discovery.py --apply with an authenticated GitHub CLI. The default invocation is read-only.