docs: adopt MIT license and Contributor Covenant 3.0

[pjeweb]

Summary

• Adds root LICENSE (MIT), matching open-mapsight/mapsight#124.
• Adds CODE_OF_CONDUCT.md at the repository root, adopting Contributor Covenant 3.0 with the standard CC 3.0 enforcement ladder — identical to open-mapsight/mapsight#126.
• Sets "license": "MIT" on the root manifest and all twelve public packages (replacing proprietary).
• Updates the README with License, Community, and Security sections aligned with the frontend monorepo.

Test plan

• Confirm LICENSE and CODE_OF_CONDUCT.md render correctly on GitHub
• Verify all composer.json files declare "license": "MIT"
• After merge, confirm Packagist reflects the updated license on the next split release