feat: sdk default <candidate> without version should set to latest LTS

[chloe41427]

Description

When calling sdk default <candidate> without specifying a version, it should set the default to the latest LTS version of that candidate. Currently, this is not working.

Expected behavior

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

Actual behavior

The command either fails or doesn't behave as expected.

Context

Related to discussion in #354 where we clarified the semantics of sdk default, sdk use, and sdk current.

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

This feature request extends sdk default to accept being called without a version argument, inferring the latest LTS.

Acceptance criteria

• sdk default java (no version) sets java to the latest LTS
• Works for all candidates with LTS releases
• Graceful error or fallback for candidates without LTS concept
• Documented in help text and usage guide

[chloe41427]

Closing in favor of a properly formatted issue using the standard template.