Prune unused mise tool versions after successful updates

[jiminu]

Summary

• Run scoped mise cleanup by default after successful tool, configuration, and Neovim plugin synchronization.
• Preserve current and tracked project versions, but exclude rollback-only tool pins during cleanup without modifying the retained CLI release or tracking records.
• Skip cleanup for failed or partial setup, configuration-only updates, unsafe paths, and failed retention checks. Dry-run does not invoke mise or write metadata.
• Document the cleanup policy and add real-mise regression coverage to CI using the pinned mise installation.

Verification

• Local: bash scripts/check.sh passed all 21 test suites, syntax checks, ShellCheck, and formatting checks.
• Real mise in temporary HOME: unused and rollback-only versions removed; current and project-required versions retained; unrelated tools and CLI rollback preserved.
• Regression checks cover tracking failures, ignored configuration settings, symlinked rollback configuration, unsafe path separators, and previous=current.
• Code review completed with the reported deletion-safety issues fixed.
• GitHub Actions results are separate from the local verification above.

Scope

PR only. No merge, release publication, or local installation.