INFRA-1344: Replace MIT licence with proprietary licence

[Rumbles]

Summary

• Replaces the MIT LICENSE with a restrictive "all rights reserved / commercial agreement required" notice that explicitly covers artifacts published to Maven Central.
• Updates the build.sbt POM licence metadata from MIT to Proprietary so artifacts published to Maven Central carry the correct licence pointer.
• Adds a Licence and use section to the README describing why the repo is public and pointing at the LICENSE.

Context

Linear: INFRA-1344

Colin White is finalising a Disclosure notice for our public GitHub repos and has classified them into:

1. Open source — community OSS, stays public under an OSS licence.
2. Commercial distribution — must stay public because the distribution mechanism requires it (SPM, CocoaPods, Maven Central); should carry a restrictive licence + a README notice that use requires a commercial agreement with Intent HQ.
3. Internal / proprietary — should be moved private.

This library belongs in Cat 2: it is published to Maven Central so that clients (notably VZW, per Alekhya Deba — confirmed still in use as of last month) can build their own integrations against the Action Processor.

The current MIT licence is the wrong posture for that — MIT actively grants third parties the right to use, modify, and redistribute. We want use to be conditional on a commercial agreement.

Slack confirmation that the repo must stay public for Maven distribution: thread (Fernando Mora, Alekhya Deba).

Outstanding confirmation

The LICENSE points licensing enquiries at `legal@intenthq.com`. Will confirm with Colin that this is the right destination before this lands.

Test plan

• Confirm `legal@intenthq.com` is the correct contact address with Colin / Ryan.
• Confirm the licence wording is acceptable to legal.
• Confirm with Fernando / Albert / Alekhya that swapping from MIT to a proprietary licence does not impact existing client integrations.