docs: refresh CLAUDE.md/ToolchainLinks currency

Correct three stale facts against the current tree: the repo IS a git
repo at top level, the Java client targets JDK 17 (not 21), and note the
NonWindows.slnx build entry point for the macOS working tree. Flag the
gradle JDK 17 toolchain requirement in ToolchainLinks.

docs/ToolchainLinks.md

@@ -123,6 +123,11 @@ typer==0.25.0
 | Gradle | 9.4.1 | `C:\Tools\gradle-9.4.1\bin\gradle.bat` |
 | Maven | 3.9.15 | `C:\Tools\apache-maven-3.9.15\bin\mvn.cmd` |
 
+The Java client was retargeted 21 -> 17 for Ignition 8.3 (`clients/java/build.gradle`
+declares a `JavaLanguageVersion.of(17)` toolchain). Gradle resolves toolchains by
+version, so a JDK 17 must be available on the build host even though the runtime
+above is Temurin 21 (a 17-targeted build still runs on 21+).
+
 Environment:
 
 ```text