A SKILL.md that mentions a support-path token which doesn't exist in the
repo (a prose glob like `references/type-*.md`, a placeholder truncated
by the regex, or a repo-only dev script) previously made GitHubSource
and UrlSource fetch fail with 'Could not fetch ... from any source',
even though every real file was present.
- _referenced_support_paths: skip glob/placeholder tokens and bare
prefix matches that can't name an actual file.
- GitHubSource/UrlSource fetch: warn and continue when a referenced
support file is missing or unfetchable, bundling what exists, instead
of aborting the whole install. Non-regular tree entries (e.g. git
symlinks) are still rejected.
Regression tests for both syntax filtering and the fetch loops.
f449374f0c