fix(claims): resolve long activity-feed slugs to canonical file stems

Activity feed emits slugs derived from PR description (the slugified claim
title), which can be longer than the on-disk file stem (agents pick shorter
hand-chosen filenames). Pure exact-stem lookup 404s on those.

Three-tier resolution in handle_claim_detail:
1. Exact stem match (existing behavior)
2. Title fallback: normalize requested slug, look up via by_title index
   (already populated from frontmatter title during _build_indexes)
3. Prefix fallback: longest common prefix among stems, anchored at 32 chars
   to prevent spurious hits

Response slug returns the canonical on-disk stem so frontend share-links
and caches converge to one form.

Repro: GET /api/claims/spacex-and-amazon-kuiper-non-endorsement-of-wef-debris-
guidelines-demonstrates-systemic-voluntary-governance-failure-at-the-scale-
where-it-matters-most was 404; now 200, returns shorter on-disk slug
'...-governance-failure'. Negative case (nonsense slug) still 404s.

Reported by Ship — Cory-facing demo path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

diagnostics/claims_api.py

@@ -423,12 +423,43 @@ async def handle_claim_detail(request):
 
     One round-trip, all data resolved server-side. Wikilinks pre-resolved.
     """
-    slug = request.match_info["slug"]
+    requested_slug = request.match_info["slug"]
     by_title, by_stem = _build_indexes()
 
+    # Resolution order: exact stem → title-normalized (handles description-derived
+    # slugs from /api/activity-feed that are longer than on-disk file stems) →
+    # stem-as-prefix (handles description-derived slugs that are shorter than the
+    # file stem because the description was truncated upstream).
+    slug = requested_slug
     rel_path = by_stem.get(slug)
     if not rel_path:
-        return web.json_response({"error": "claim not found", "slug": slug},
+        # Title fallback: requested slug = slugified frontmatter title
+        norm = _normalize_for_match(requested_slug)
+        resolved_stem = by_title.get(norm)
+        if resolved_stem:
+            slug = resolved_stem
+            rel_path = by_stem.get(resolved_stem)
+    if not rel_path:
+        # Prefix fallback: walk stems sharing a common prefix with the request,
+        # pick longest match. Anchored at 32 chars to avoid spurious hits.
+        norm_req = _normalize_for_match(requested_slug)
+        best_stem = None
+        best_len = 0
+        for stem in by_stem:
+            norm_stem = _normalize_for_match(stem)
+            common = 0
+            for a, b in zip(norm_req, norm_stem):
+                if a != b:
+                    break
+                common += 1
+            if common >= 32 and common > best_len:
+                best_stem = stem
+                best_len = common
+        if best_stem:
+            slug = best_stem
+            rel_path = by_stem.get(best_stem)
+    if not rel_path:
+        return web.json_response({"error": "claim not found", "slug": requested_slug},
                                  status=404, headers=CORS_HEADERS)
 
     filepath = CODEX_BASE / rel_path