Address CodeRabbit review on #3771:
- Treat immediately empty (`---\n---`) and whitespace-only frontmatter
blocks as empty metadata instead of throwing or leaking the delimiters
into the installed body.
- Reject an empty `paths: []` list before serialization so an installed
instruction never receives an empty `applyTo:` glob.
- Rewrite relative Markdown links that resolve inside the installed rule
set to their renamed `.instructions.md` targets; links escaping rules/,
external URLs, absolute paths, anchors, and README.md stay unchanged.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>