Expand README with quick start, delivery guarantees, and module diagram (KOJAK-33)

[endrju19]

Summary

• Expanded README from 87 to 147 lines with SML portfolio conventions
• Added badges: Discourse, CI, dynamic Kotlin version (reads from libs.versions.toml), JVM 21, License
• Added Quick Start (Spring Boot) with 3 code blocks: BOM dependencies, HttpMessageDeliverer bean, SpringOutboxPublisher usage with constructor injection
• Added "How It Works" section with delivery guarantees (at-least-once, concurrent processing, error classification)
• Added Mermaid module dependency diagram + expanded modules table (now includes okapi-mysql)
• Added Contributing section (ox pattern, adapted for Kotlin/Gradle), Project sponsor, Copyright
• Removed standalone usage section (deferred to KOJAK-40 when okapi-ktor is available)

Test plan

• Verify Mermaid diagram renders on GitHub
• Verify dynamic Kotlin badge shows correct version
• Verify all links are reachable
• Verify badge images load