fix: preserve local metadata during upstream sync

[hwj123hwj]

The scheduled upstream sync can conflict when upstream changes a SKILL.md version while this repository adds marketplace metadata to the same YAML frontmatter.

This change:

• performs conservative field-level three-way merging for SKILL.md frontmatter;
• preserves local category/tags/upstream metadata while taking independent upstream version/body changes;
• leaves true same-field/body conflicts for review;
• prevents the validator from masking malformed frontmatter with NoMethodError;
• documents the ownership policy and adds a regression self-test.