Feature: sdk default <candidate> should set to latest LTS when version omitted #433

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Feature request

Currently, sdk default <candidate> requires an explicit version argument:

sdk default java 21.0.1

This feature request adds the ability to call sdk default <candidate> without a version, which should automatically set the default to the latest LTS version of that candidate:

sdk default java
# Should set java to the latest LTS (e.g., 21 or 25)

Why this would be useful

• Convenience: Users don't need to look up the latest LTS version manually
• Consistency with bash SDK: Aligns with expected SDKMAN! behavior
• Workflow improvement: Simplifies automated setup scripts that want to default to LTS

Related

Discussion in #354 about sdk default, sdk use, and sdk current semantics clarified this gap.

• sdk default <candidate> <version> — sets the global default to a specific version
• sdk use <candidate> <version> — sets version for this shell session only
• sdk current <candidate> — reports currently active version in this shell

This feature extends sdk default to infer the latest LTS when no version is provided.