Fix BOM-prefixed Windows cleanup selection JSON

[rodion-m]

Summary

• accept BOM-prefixed and BOM-less UTF-8 cleanup-selection JSON
• use BOM-safe decoding for cleanup-plan scan data
• add regression coverage for loading and dry-run behavior
• bump the plugin manifests to 5.0.1 as required for skill asset changes

Root cause

Windows PowerShell and some editors can emit a UTF-8 BOM. Reading with encoding="utf-8" preserves the leading marker, which causes json.loads to reject otherwise valid JSON. utf-8-sig removes the marker when present and behaves like UTF-8 when it is absent.

Safety

• based directly on the rewritten, sanitized main
• contains no history from the old fork branch
• does not contain skills/agents-consilium/prompt.md or prompt.txt
• preserves the existing validation and dry-run path

Validation

• python3 skills/maintaining-windows-health/assets/test_validate_command.py — 56 passed, 0 failed
• python3 -m py_compile for all three changed Python files
• git diff --check
• gitleaks dir — no leaks found
• plugin and marketplace manifests parsed with jq

Supersedes #2 while preserving the original contributor's implementation and authorship.

Fixes #1

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[rodion-m]

Closing this replacement: the maintainer will preserve and merge the original contributor PR #2 instead.