Add Posnic as open source POS and billing software

Following CONTRIBUTING_TO_OSB.md, please add:

Add open source tool: Posnic
- Repository: https://github.com/Posnic/POS
- Website: https://www.posnic.com/
- Description: Free, offline-first open source POS and billing software for retail shops and restaurants. Its primary database and API run locally on the till, with optional self-hosted and online/offline workflows.
- Category: E-commerce Platforms
- Features: Offline-first checkout, billing and receipts, inventory management, barcode scanning, returns and part payments, restaurant mode, reports, local backups, self-hosted option

## Additional information

- License: AGPL-3.0-only for Posnic-owned source; release packages include separately licensed components
- Current stable release: v1.6.1
- Platforms: Windows, macOS and Linux
- Download: https://github.com/Posnic/POS/releases/tag/v1.6.1
- Suggested narrower category if useful: Retail POS / Point of Sale

Posnic's local edition has a zero software price and no trial clock. The optional paid Posnic Cloud service is separate and provides multi-till sync, off-site backups and remote dashboards; it is not required for local checkout.

Disclosure: I am submitting this on behalf of the Posnic project. I used automation to prepare the submission and checked the statements against the current public repository and release.

Start by reading CONTRIBUTING_TO_OSB.md, then locate the catalog data or listing entry point used for software submissions. Add Posnic with the repository, website, description, category, features, license, release, platforms, and download details provided here. Done means the listing follows the repository format and preserves the distinction between the local edition and optional cloud service.

Written by the indexing model from the issue text.