feat(okf): resource-URL https allowlist reject-gate (T3, TDD, +10)

src/llm_ingestion_guard/okf.py

@@ -31,9 +31,11 @@ __all__ = [
     "parse_frontmatter",
     "scan_concept",
     "validate_concept_path",
+    "validate_resource_url",
     "OKFError",
     "OKFFrontmatterError",
     "OKFPathError",
+    "OKFResourceError",
 ]
 
 _FENCE = "---"
@@ -58,6 +60,13 @@ class OKFPathError(OKFError):
     """A concept path is unsafe (traversal, absolute, or reserved-name shadow)."""
 
 
+class OKFResourceError(OKFError):
+    """A ``resource`` URL is not on the https allowlist."""
+
+
+_URL_SCHEME_RE = re.compile(r"^([A-Za-z][A-Za-z0-9+.\-]*):")
+
+
 # `index.md` (directory listing) and `log.md` (update history) are reserved by
 # the OKF spec and MUST NOT name concept documents — at any directory level.
 _RESERVED_BASENAMES = frozenset({"index.md", "log.md"})
@@ -159,6 +168,33 @@ def validate_concept_path(path):
     return path[: -len(".md")]
 
 
+def validate_resource_url(url):
+    """Validate a concept's ``resource`` URL against the https allowlist (T3).
+
+    The OKF format places no constraint on the ``resource`` scheme (verified
+    against SPEC.md), so this default-deny allowlist is the only gate: it
+    **rejects** anything that is not ``https`` — ``http``, ``data:``,
+    ``javascript:``, ``file:``, ``blob:``, ``ftp:`` and schemeless/relative
+    strings — *before commit*. This is reject, not defang: ``neutralize`` renders
+    dangerous schemes inert for human audit; this refuses to persist them at all.
+
+    Returns ``url`` unchanged on success; raises :class:`OKFResourceError`
+    otherwise.
+    """
+    if not url or not isinstance(url, str):
+        raise OKFResourceError("empty or non-string resource URL: %r" % (url,))
+    stripped = url.strip()
+    if " " in stripped or any(ord(c) < 0x20 for c in stripped):
+        raise OKFResourceError("resource URL contains whitespace/control chars: %r" % url)
+    match = _URL_SCHEME_RE.match(stripped)
+    scheme = match.group(1).lower() if match else None
+    if scheme != "https":
+        raise OKFResourceError(
+            "resource URL must use the https scheme (got %r): %r" % (scheme, url)
+        )
+    return url
+
+
 def _parse_flat(fm_lines):
     result = {}
     i = 0