Add onset/duration metadata fields and remove incorrect custom field

Added essential BIDS metadata definitions for event JSON sidecars:
- onset: Event onset time in seconds
- duration: Event duration in seconds

These were needed because annotation JSON sidecars contain these fields
and the validator requires metadata definitions for all JSON fields.

Removed incorrectly added shot_number field which is a custom annotation
column specific to examples, not a general BIDS field.

Tests: Basic stimuli validation now works with proper schema definitions

Author: neuromechanist

src/schema/objects/metadata.yaml

@@ -4218,3 +4218,20 @@ type:
     Type of stimulus file (for example, image, audio, video, audiovideo).
     Refers to the suffix of the stimulus file when present in stimulus metadata.
   type: string
+duration:
+  name: duration
+  display_name: Duration
+  description: |
+    Duration of the event (measured from onset) in seconds.
+    Must always be either zero or positive (or `n/a` if unavailable).
+  type: number
+  unit: s
+  minimum: 0
+onset:
+  name: onset  
+  display_name: Onset
+  description: |
+    Onset (in seconds) of the event, measured from the beginning of the acquisition
+    of the first data point stored in the corresponding task data file.
+  type: number
+  unit: s