core: type agent timestamps

crates/crank-registry/src/model.rs

@@ -170,9 +170,9 @@ pub struct AgentSummary {
     pub status: AgentStatus,
     pub current_draft_version: u32,
     pub latest_published_version: Option<u32>,
-    pub created_at: String,
-    pub updated_at: String,
-    pub published_at: Option<String>,
+    pub created_at: OffsetDateTime,
+    pub updated_at: OffsetDateTime,
+    pub published_at: Option<OffsetDateTime>,
 }
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
@@ -181,7 +181,7 @@ pub struct AgentVersionRecord {
     pub workspace_id: WorkspaceId,
     pub version: u32,
     pub status: AgentStatus,
-    pub created_at: String,
+    pub created_at: OffsetDateTime,
     pub snapshot: AgentVersion,
     pub bindings: Vec<AgentOperationBinding>,
 }
@@ -419,7 +419,7 @@ pub struct PublishAgentRequest<'a> {
     pub workspace_id: &'a WorkspaceId,
     pub agent_id: &'a AgentId,
     pub version: u32,
-    pub published_at: &'a str,
+    pub published_at: &'a OffsetDateTime,
     pub published_by: Option<&'a str>,
 }