Prepare 1.9.0 release: dep bumps and CI documentation

[pambrose]

Summary

Bumps the project version to 1.9.0 and records the release across the documentation set. No DSL, content, or runtime source changes — this touches only the version catalog, gradle.properties, and docs.

Dependency changes

Net versus the 1.8.0 tag:

Library	1.8.0	1.9.0
readingbat-core / readingbat-kotest	3.2.1	3.3.0
Kotlin	2.4.0	2.4.10
Kotest	6.2.1	6.2.3
core-utils	2.9.3	3.2.1
Kotlinter	5.5.0	5.6.0

core-utils is a major bump of a library used directly by this template — FileSystemSource, GitHubRepo, and OwnerType.Organization in Content.kt, and toDoubleQuoted in ContentTests.kt. All four call sites compile unchanged against 3.x.

Also renames the [versions] catalog key ben-manes-versions → versions. The plugin alias remains ben-manes-versions, so alias(libs.plugins.ben.manes.versions) in build.gradle.kts is unchanged.

Note

This partially reverses a 1.8.0 change, which renamed the key the other direction to avoid colliding with the [versions] table name. Nothing breaks either way, but worth a look if that revert was unintentional.

Documentation

• CHANGELOG.md — 1.9.0 section plus the compare link
• RELEASE_NOTES.md — v1.9.0 highlights, dependency table, upgrade notes, verification results
• CLAUDE.md / README.md — new "Continuous Integration" sections documenting the GitHub Actions workflow added in 8346cac
• README.md — CI status badge
• llms.txt — refreshed readingbat-core and current-version values, CI noted in Key Files and Tech Stack

Test plan

Run locally on JDK 25 before the docs were written:

• ./gradlew --rerun-tasks check — BUILD SUCCESSFUL; all three ContentTests specs pass
• ./gradlew buildFatJar — BUILD SUCCESSFUL; produces build/libs/server.jar
• ./gradlew lintKotlin detekt — both clean
• build/libs/readingbat-template-1.9.0.jar confirms the version bump took effect

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