Feature: sdk clean subcommand #397

• I have read the CONTRIBUTING guidelines
• I've had a conversation on our community Discord server.

Feature request

Implement a sdk clean subcommand that removes installed SDK versions no longer referenced by any .sdkmanrc file — i.e., orphaned versions that have accumulated over time.

This is analogous to brew autoremove for SDKMAN! candidates: it identifies versions that are installed but not pinned by any project configuration, and removes them to recover disk space.

Proposed behaviour

• Scan all .sdkmanrc files found under a configurable base directory (e.g. ~/)
• Collect the union of all candidate versions referenced
• Compare against all currently installed versions
• Prompt the user to confirm removal of any installed versions not referenced by any .sdkmanrc
• Optionally support a --dry-run flag to list what would be removed without acting
• Always protect the current default version of each candidate, even if it is not referenced by any .sdkmanrc file — so if .sdkmanrc pins java=21.0.10-tem and the default is 17.0.18-tem, both are kept; only versions that are neither referenced nor the default are candidates for removal

Note: this is intentionally separate from sdk flush, which handles temporary files and caches under $SDKMAN_DIR/tmp and $SDKMAN_DIR/var. sdk clean operates on installed candidate versions only.

Originally requested in sdkman/sdkman-cli#688.