fix(claims): unwrap ```markdown code fences + 404 for fragments

Two issues Ship hit on the Montreal Protocol claim:

1. 500 on canonical stem lookup. File starts with ```markdown wrapper
   instead of bare --- frontmatter delimiter. _split_frontmatter checked
   startswith("---") and bailed, returning "frontmatter parse failed".
   Same wrapper exists on 6 other claim files (audit grep). Now strip
   the wrapper before frontmatter detection.

2. 404 on long activity-feed slug. Same root cause — _build_indexes
   couldn't read the file's title from frontmatter, so by_title never
   indexed it, so title-fallback resolution had nothing to match against.
   Both bugs collapse once we unwrap.

Also: switched "file exists but has no frontmatter" from 500 to 404 with
reason=file_no_frontmatter. These are stray enrichment fragments living
in domains/ that never got merged into a parent claim. From the API
caller's perspective there's no claim at that slug — 500 implied
"server bug, retry later" which isn't actionable.

Verified: 3/3 wrapped claims (montreal, medicare, dod) now return 200
warm-cache ~13ms. Long-slug repro (montreal) resolves via title fallback
to canonical stem. Negative test (nonsense slug) still 404.

diagnostics/claims_api.py

@@ -68,8 +68,22 @@ def _normalize_for_match(s):
 
 # ─── Frontmatter parse ─────────────────────────────────────────────────────
 
+_CODE_FENCE_WRAPPER_RE = re.compile(r"^\s*```(?:markdown|md)?\s*\n(.*?)\n```\s*$", re.DOTALL)
+
+
 def _split_frontmatter(text):
-    """Return (frontmatter_dict, body_str) or (None, None) if not a claim file."""
+    """Return (frontmatter_dict, body_str) or (None, None) if not a claim file.
+
+    Tolerates files wrapped in a top-level ```markdown ... ``` code fence —
+    some agents have produced these (e.g. Montreal Protocol claim from Astra,
+    2024-12-09). Unwrap once before frontmatter detection.
+    """
+    if not text:
+        return None, None
+    m = _CODE_FENCE_WRAPPER_RE.match(text)
+    if m:
+        text = m.group(1)
+    text = text.lstrip()
     if not text.startswith("---"):
         return None, None
     try:
@@ -465,8 +479,13 @@ async def handle_claim_detail(request):
     filepath = CODEX_BASE / rel_path
     fm, body = _read_claim_file(filepath)
     if not fm:
-        return web.json_response({"error": "frontmatter parse failed", "slug": slug},
-                                 status=500, headers=CORS_HEADERS)
+        # File exists at this stem but has no parseable frontmatter — almost
+        # always a stray enrichment fragment that landed in domains/ without
+        # being merged into a parent claim. Surfacing as 404 (no claim here)
+        # not 500: the caller can't act on it differently anyway.
+        return web.json_response({"error": "claim not found", "slug": slug,
+                                  "reason": "file_no_frontmatter"},
+                                 status=404, headers=CORS_HEADERS)
 
     # Open read-only DB connection for this request
     conn = sqlite3.connect(f"file:{DB_PATH}?mode=ro", uri=True)