Fix stimuli catalog files to use stem pattern

Change stimuli_table and annotations_table definitions from
suffix-based to stem-based pattern, matching how participants.tsv
is handled. Stimuli catalog files are not entity-suffixed files
but special root-level catalog files per BEP044 specification.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: neuromechanist

src/schema/rules/files/raw/stimuli.yaml

@@ -3,8 +3,7 @@
 stimuli_table:
   level: recommended
   path: stimuli
-  suffixes:
-    - stimuli
+  stem: stimuli
   extensions:
     - .json
     - .tsv
@@ -15,8 +14,7 @@ stimuli_table:
 annotations_table:
   level: recommended
   path: stimuli
-  suffixes:
-    - annotations
+  stem: annotations
   extensions:
     - .json
     - .tsv