Separate metadata fields from TSV columns in events rules

Move stim_file and stim_id from JSON sidecar rules to TSV column
rules where they belong. Sidecar rules should only contain JSON
metadata fields, while tabular rules contain TSV columns.

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

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

Author: neuromechanist

src/schema/rules/sidecars/events.yaml

@@ -12,5 +12,3 @@ StimulusPresentation:
   fields:
     StimulusPresentation: recommended
     VisionCorrection: optional
-    stim_file: recommended
-    stim_id: recommended

src/schema/rules/tabular_data/events.yaml

@@ -9,6 +9,7 @@ Events:
     response_time: optional
     HED: optional
     stim_file: optional
+    stim_id: optional
     channel:
       level: optional
       description_addendum: |