chore: change license from MIT to AGPL-3.0 + Commercial dual license

[q-soriarty]

Summary

Change the project license from MIT to dual AGPL-3.0 + Commercial license.

Motivation

• Community users: free to use under AGPL-3.0 terms
• Commercial use (e.g., Copia integrating into their platform): requires a separate commercial license agreement with the author
• Protects the project from being incorporated into proprietary products without contribution back

Scope

Retroactive application

Since the project currently has:

• 0 stars, 0 forks
• Single contributor (sole copyright holder)
• No external contributions

The license change can be applied retroactively to all versions. The sole copyright holder has the right to relicense at any time.

Tasks

• Replace LICENSE file content from MIT to AGPL-3.0
• Add LICENSE-COMMERCIAL.md explaining commercial licensing terms and contact
• Update README.md license section
• Update CLAUDE.md if it references MIT
• Update .goreleaser.yml license field
• Update go.mod or any file referencing MIT
• Add license header to all source files (optional but recommended for AGPL)
• Update CONTRIBUTING.md with CLA requirement for future contributors
• Tag all existing GitHub releases with updated license notice
• Consider adding NOTICE file for attribution

Commercial license

The commercial license allows usage without AGPL obligations (no source disclosure requirement). Terms to be negotiated individually. Contact: [to be defined]

Reference

Similar dual-license projects:

• MongoDB (SSPL + Commercial)
• Grafana (AGPL + Commercial)
• Elasticsearch (SSPL + Elastic License)

[github-actions]

Closed by #137 (merged into develop).