skills hub: don't abort installs on missing referenced support files

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.

tests/tools/test_skills_hub.py

@@ -1918,3 +1918,70 @@ class TestLoadHermesIndex:
 
         data = hub._load_hermes_index()
         assert data == {"skills": [{"name": "stale"}]}
+
+
+# ---------------------------------------------------------------------------
+# Referenced-path extraction & missing support files (regression: a prose
+# glob or a repo-only dev tool referenced in SKILL.md must not abort install)
+# ---------------------------------------------------------------------------
+
+
+class TestReferencedSupportPaths:
+    def test_ignores_globs_placeholders_and_truncated_tokens(self):
+        md = (
+            "Load `references/type-*.md` before drawing, and "
+            "`references/type-<name>.md` for the matching type.\n"
+            "Real files: [flowchart](references/type-flowchart.md) and "
+            "`references/type-architecture.md`.\n"
+        )
+        assert _referenced_support_paths(md) == {
+            "references/type-flowchart.md",
+            "references/type-architecture.md",
+        }
+
+    def test_keeps_real_backtick_references(self):
+        md = "Run `python3 scripts/self_check.py <file>` and see `references/guide.md`.\n"
+        assert _referenced_support_paths(md) == {
+            "scripts/self_check.py",
+            "references/guide.md",
+        }
+
+
+class TestGitHubSourceFetchMissingReferencedFile:
+    def _source(self):
+        auth = MagicMock(spec=GitHubAuth)
+        return GitHubSource(auth=auth)
+
+    def test_fetch_skips_missing_referenced_file_and_keeps_the_rest(self):
+        md = (
+            "---\nname: demo\ndescription: demo\n---\n\n"
+            "See [guide](references/guide.md) and `references/missing.md`.\n"
+        )
+        tree_entries = [
+            {"path": "skills/demo/references/guide.md", "type": "blob", "mode": "100644"},
+        ]
+        source = self._source()
+        with patch.object(source, "_fetch_file_content", return_value=md), \
+             patch.object(source, "_get_repo_tree", return_value=("main", tree_entries)), \
+             patch.object(source, "_fetch_file_bytes", return_value=b"# guide"):
+            bundle = source.fetch("owner/repo/skills/demo")
+
+        assert bundle is not None
+        assert bundle.name == "demo"
+        # The present referenced file is bundled…
+        assert bundle.files["references/guide.md"] == b"# guide"
+        # …and the missing one is warned about and skipped, not fatal.
+        assert "references/missing.md" not in bundle.files
+
+
+class TestUrlSourceFetchMissingReferencedFile:
+    def test_fetch_skips_missing_referenced_file(self):
+        md = "---\nname: demo\ndescription: demo\n---\n\nSee `references/missing.md`.\n"
+        source = UrlSource()
+        with patch.object(source, "_fetch_text", return_value=md), \
+             patch.object(source, "_fetch_bytes", return_value=None):
+            bundle = source.fetch("https://example.com/skills/demo/SKILL.md")
+
+        assert bundle is not None
+        assert bundle.name == "demo"
+        assert "references/missing.md" not in bundle.files

tools/skills_hub.py

@@ -209,6 +209,12 @@ def _referenced_support_paths(skill_md: str) -> Optional[set[str]]:
         except ValueError:
             return None
         if safe.split("/", 1)[0] in _ALLOWED_SUPPORT_DIRS:
+            # Prose globs/placeholders — e.g. ``references/type-*.md`` or
+            # ``references/type-<name>.md`` (which the regex truncates to the
+            # bare prefix ``references/type-``) — are agent instructions, not
+            # files. Only tokens that can name an actual file are references.
+            if re.search(r"[*?<>]", safe) or "." not in safe.rsplit("/", 1)[-1]:
+                continue
             paths.add(safe)
     return paths
 
@@ -733,7 +739,9 @@ class GitHubSource(SkillSource):
                     return None
                 content = self._fetch_file_bytes(repo, item_path)
                 if content is None:
-                    return None
+                    logger.warning("Failed to fetch referenced skill support "
+                                   "file; continuing without it: %s", item_path)
+                    continue
                 files[rel_path] = content
             # A support file SKILL.md links must actually exist as a regular
             # file — a missing or symlinked referenced path rejects the
@@ -750,7 +758,9 @@ class GitHubSource(SkillSource):
             for rel_path in referenced:
                 content = self._fetch_file_bytes(repo, f"{skill_path.rstrip('/')}/{rel_path}")
                 if content is None:
-                    return None
+                    logger.warning("Failed to fetch referenced skill support "
+                                   "file; continuing without it: %s", rel_path)
+                    continue
                 files[rel_path] = content
             revision = ""